Flatline - to produce a flat evaluation curve.
As I understand engine chess, engines "flatline" in fortress positions. In these positions there are a lot of effectively-equivalent positions that return the same static eval. For example, in a KR vs KB endgame, almost all positions are the same, so it's not surprising that the eval flatlines.
NNUE however is different, because it's a neural network. It's hard for me to imagine that all the computations that's needed in a NN can yield exactly the same number for many different positions. That's why for example Leela Chess Zero never flatlines. How, then, can Stockfish NNUE flatline?
Example of a game where Stockfish NNUE flatlines: first from move 24 to 32 at 0.13, and then from move 33 to 38 at 0.05.