Depth

In analysis with a chess engine, a program that calculates positions and compares continuations, depth indicates how far one stage of its search has progressed. One useful term is ply: a ply is one move by one side. A White move followed by a Black move therefore makes two plies. In a simple search, depth 10 would correspond to looking ten plies ahead.

Modern engines do not examine every branch of the game tree to one uniform distance. The game tree is the branching set of legal continuations growing from a position. To save time, an engine may discard unpromising branches, search some branches less deeply, and extend others that appear important. In Stockfish, the displayed depth is technically a counter for successive search passes starting from the analyzed position, so it does not mean every continuation was calculated to exactly that many plies.

A higher depth usually means the same engine has had a chance to revisit the position with a more mature search, but it does not guarantee that its evaluation of which side is better is correct. Depth should also be distinguished from the visible length of the principal variation, the line the engine considers best, and from nodes per second, the number of positions it processes each second. Comparing an isolated depth number across different engines or settings can be misleading because their search methods are not identical.