In chess, a combination is a sequence of moves, often initiated by a sacrifice, which leaves the opponent few options and results in tangible gain.
At most points in a chess game, each player has several reasonable options from which to choose, which makes it difficult to plan ahead except in strategic terms.
In 1952/53, the editors of Shakhmaty v SSSR decided on this definition: A combination is a forced sequence of moves which uses tactical means and exploits specific peculiarities of the position to achieve a certain goal.
A combination is a blend of ideas – pins, forks, discovered checks, double attacks – which endow the pieces with magical power.
A combination is usually built out of two or more fundamental chess tactics such as forks, pins, skewers, undermining, discovered attacks, etc.