Rotation is the movement of a player or a group of players from one point on the map to another in response to information about the opponent's actions. The word comes from the English "rotation" ("turning," "changing") and is used the same way in both CS2 and Dota 2, although its essence differs slightly between the games.
In CS2, rotation more often refers to the defending side moving from one bombsite to another once it becomes clear where the attacking team is actually pushing. For example, if players hear footsteps or see on the radar that the opponent is massing on site B rather than A, part of the lineup rotates there to take up positions before the bomb detonates or before the retake begins — the attempt to reclaim a site with the bomb already planted. The speed and timing of such a rotation directly affect the outcome of the round: rotating too early leaves the second site empty, while rotating too late means not making it in time.
In Dota 2, rotation usually means a hero moving from one lane to another or coming out onto the map to help allies in a different part of the field. For example, a support hero might rotate from their lane into the enemy jungle to set up an ambush (gank), or move to an enemy tower for a joint attack. Such decisions are often made by the team's captain (IGL), based on the map and information from allies.
Rotation is closely tied to understanding timing and sound on the map: a player who doesn't hear footsteps or isn't watching the minimap ends up rotating either too late or into empty space. That's why commentators and analysts often discuss rotation separately from pure shooting mechanics — it's more an element of team game-reading than of individual skill.
In a broader sense, the term is also used outside the context of a specific round — for example, when talking about a team's roster rotation between tournaments or map rotation in a tournament bracket. But within the game itself, "rotation" almost always means movement across the map in response to the opponent's actions.