Definition
The body adapts to the exact demand placed on it, so training should look like the goal: heavy for strength, longer sets for size, the sport's own pattern for sport.
SAID stands for Specific Adaptation to Imposed Demands. A client who wants to squat 140 kg has to squat heavy; leg presses help but do not get them there. A runner's lifting should support running, not replace it. A general fitness goal gets a general programme.
Specificity rises as the goal gets closer, which is the logic of block periodization: general work early, specific work late.
Too specific too early causes overuse and boredom, so most programmes balance it with variation: the same pattern, a different exercise, for a block.