Is a bad bishop a bishop that is trapped by your own pawns? (For example, all your pawns are on dark squares, so the dark squared bishop is bad.) Or is it when your opponent's pawns trap your bishop's mobility? (For example, they control the dark squares which your bishop controls, so the bishop is bad).
When evaluating a bishop, there are two totally separate things that must be considered. The first is whether a bishop is "good" or "bad". These are very much misnomers because they have nothing to do with the worth of the bishop. The second consideration is whether the bishop is "active" or "inactive". Finally, there may be mitigating factors as well (is the bishop doing something useful?).
Good vs Bad
A good bishop is on the opposite color as its pawns. A bad bishop is on the same color as its pawns. For example if the white pawn structure is c3, d4, e3, f4, then the light squared bishop is the good bishop and the dark squared bishop is the bad bishop.
Active vs Inactive
An active bishop is outside of the friendly pawn chain (in the above example, if the bishop is on e5, then it is clearly both active and bad). An inactive bishop is trapped by pawn chains - either the player's color or the opponent's.
There are two main mitigating factors. The first can be remembered by a common phrase: "bad bishops defend good pawns". This means that a bishop might be very bad (same color as all the friendly pawns) but if it is defending them from attack in the endgame, then it might be enough to hold a draw. Usually it is much harder to win with the bad bishop though.
The other main mitigating factor is if the bishop is defending its own passed pawn or stopping an enemy passed pawn.
Several useful answers were given here last year, answers which illuminate the exceptions to the rule so well that—when the answers are read together—they seem almost to leave the rule itself in shadow. Let me cast my flickering light therefore directly upon the rule.
The good and the bad bishop
The technical adjectives "good" and "bad" have much to do with the worth of a bishop. A good bishop very often wins a game of chess. Indeed, unless one has a material lead of two pawns or better, has a passed pawn, or can push or trade pawns to make a bad bishop better, to win an endgame with the bad bishop can be hard. On my own, limited experience, I should go further: I would say that to win with the bad bishop, without a strong counter-advantage to compensate so great a disadvantage, were seldom possible absent a gross enemy blunder.
Good against bad bishops in the endgame
Endgames that feature opposing bishops on squares of the same color are quite common in chess, as you undoubtedly know. As soon as such an endgame seems probable, one might consider pushing as many of one's pawns as possible, on both flanks, as fast as possible, to squares of the color the bishops cannot reach.
In class-D and class-C play, to have pushed pawns in so simple, innocuous-seeming a manner often wins the game 20 or so moves later; insofar as opponents at this level, not grasping the principle, have no idea why you would spend valuable late-middlegame time to push seemingly random pawns. One, quiet attraction of winning a game in this manner is that it leaves one's opponent with little notion as to why he has lost. (Indeed, I suspect that it is in large measure because of the mysterious bad-bishop principle that class-D players tend to dislike endgames, why some will launch unsound attacks just to avoid endgames. They repeatedly lose endgames for no visible reason and don't know why. All endgame maneuvering, all endgame calculation seems mysteriously futile to them. The unseen reason, of course, is that they have lost, or at best drawn, before the endgame ever began by letting opponents choose during the middlegame the color of the squares on which endgame pawns must stand. They think you and your bishop cleverly nimble, hardly realizing how easy it is to be nimble when one's bishop is good.)
Other endgames involving bishops
Endgames that feature opposing bishops on squares of different colors admittedly work on another principle, though such games are significantly less common in practical play. In bishop-against-knight endgames, which are common enough, however, the square-color principle again emerges, though in a less pure form.
Why the good bishop is good
The good bishop is good for three principal reasons:
- because it guards and attacks squares its pawns do not cover;
- because it is nimble; and
- because its pawns can blockade, and thus fix, opposing pawns on bad squares.
The bad bishop does possess the counter-advantage that it and its pawns can guard one another but, except maybe when the pawn is a rook pawn, such a counter-advantage seldom compensates.
Definitions and remarks
Others have already answered your question about definitions well enough, so here I will add only that I have read the definition as given both with and without reference specifically to central pawns. When speaking of the opening, I should call a bishop technically bad when central pawns stood on squares of its color, without much reference to flank pawns, as others have noted. However, from the early middlegame, my view should gradually broaden toward the flanks, eventually taking all pawns into account of the bad bishop. The two definitions are not unrelated, of course, for the technically bad bishop of the opening is more likely to become the dispositively bad bishop of the endgame than is a technically good bishop, unless its owner either has taken steps to improve it or has swapped it off.
Consider establishing a good bishop when you can. It may win you some games otherwise drawn or lost.
A "bad" bishop is one that is not performing the normal function of a bishop. That is usually one behind its own pawns. The test is, "if I replaced the bishop with a pawn, would it make any difference?" If the answer is no, then it is a bad bishop.
A Bishop can be behind a wall of pawns and still be very useful. Say there is a bishop on g7 in front of its king on g8, and behind pawns on h6, g5, and f6. As the "linchpin" of the defensive system, the bishop is very useful, even though it has little mobility. (Unless this is a situation where all the action is on the queenside, in which case the bishop would really be "bad.")
The term itself is rather simple. A "Bad Bishop" is one that is on the same color as most/all ( the higher the percentage, the worse the bishop) and is behind them, that is to say, the pawns are preventing it from attacking the enemy.
If you can get your light-square bishop on the outside of your pawns, it's will no longer be bad, though it may have reduced mobility.
Defensive or not, a bad bishop is bad (the logical end of defensive warfare is surrender - Napoleon) and its bad because it does nothing to limit, or otherwise influence, the actions of the enemy.
Never willingly accept a bad bishop unless you get something in return.
A bad bishop is one where many of its own pawns are on the same color as it. This definition is very strict, and there's very little subjectivity involved.
An inactive bishop is one that lacks activity. This definition is much less strict, allowing some subjectivity.
As per your question, a bishop trapped by its own pawns would be both bad and inactive, while a bishop trapped by its own pawns would just be inactive (we need info on the color squares of its own pawns to say whether it's "good" or "bad").
It should be noted that a bishop can be "good" and "inactive". Good meaning its own pawns are on the other color complex as it, but inactive since it lacks activity (possibly due to the opponent's pawns blocking it in).
Every piece has a strength that depends on the position on the board. More specifically, it depends on the pawn structure and piece placement. As you correctly point out, a bishop trapped behind its own pawns (e.g. white has Bc1, pawns on b4, c3 and d2; black has pawns on b5, c4 and d3) is a bad bishop. In general, a bishop that has almost no available squares and/or no targets to attack is a bad bishop. For example, the rule of placing your pawns on squares that are of the opposite color to the squares that the bishop controls is related to both of these factors. If you have a dark squared bishop and you place your pawns on light squares, two things occur.
- Your bishop is not blocked by your own pawns.
- Your opponent will place pieces and pawns on the same color as your bishop controls.
Another example of a bad bishop is a bishop that has to support its own pawn chain, e.g. white has Bb3 and pawns on c2 and a4. As soon as the bishop moves, both pawns become unprotected. A bishop needs
- Responsibility to protect no pawns or only the pawn base (e.g. c2 in a c2-b3-a4 chain).
- Targets to attack (e.g. white has Be3 and pawn on a6; black has pawns on a7 and b6; Bxb6 is a constant threat, as well as Be3-f4-b8xa7).
The less these conditions hold true, the worse bishop you get!
One way "bad bishop" can be defined and avoided is by using Capablanca's rule. 1) If you have a bishop keep most of your pawns on the squares of opposite color. This rule has priority. 2) If opponent has a bishop, limit it by using your pawns. Of course some factors might balance these rules out.
The article below seems to illustrate this very nicely. It shows how a strong player makes a decision about the "quality" of their bishop and based on that decides on the best structure. https://www.chess.com/blog/Illingworth/capablancas-rule
Bad bishops and doubled pawns are both commonly misunderstood. The advice to avoid them is usually based on very superficial observations. If you were to focus your efforts on properly understanding them you should gain a class at least. And understanding takes high precedence over defining.
Look at The Secret Life of Bad Bishops by Esben Lund from Quality Chess