Evaluation function
An evaluation function, also known as a heuristic evaluation function or static evaluation function, is a function used by game-playing programs to estimate the value or goodness of a position in the minimax and related algorithms. The evaluation function is typically designed to prioritize speed over accuracy; the function looks only at the current position and does not explore possible moves (therefore static).
In chess
One popular strategy for constructing evaluation functions is as a weighted sum of various factors that are thought to influence the value of a position. For instance, an evaluation function for chess might take the form
c1 * material + c2 * mobility + c3 * king safety + c4 * center control + ...
Such as
f(P) = 9(Q-Q') + 5(R-R') + 3(B-B'+N-N') + (P-P') - 0.5(D-D'+S-S'+I-I') + 0.1(M-M') + ...
in which:
Q, R, B, N, P are the number of white queens, rooks, bishops, knights and pawns on the board.
D, S, I are doubled, backward and isolated white pawns.
M represents white mobility (measured, say, as the number of legal moves available to White).