Variation

A series of moves logically connected to one another.

A variation can branch in different directions depending on the opponent's replies. Calculating variations quickly, and being able to foresee every possible branch, matters enormously for achieving a successful result in the game.

In openings

The word "variation" is also widely used in openings. Here, though, the concept is broader: the word sometimes refers to an entire opening system — for example, the Chigorin Variation of the Ruy Lopez:

In composition

In composition, a distinction is drawn between the main variation, thematic variations that reveal the core idea of the problem or study, and side variations.