Mid (from the English "middle") is the central part of the map that connects site A and site B. Unlike side corridors that lead to only one site, mid almost always provides visibility onto several directions at once and allows a quick switch from one flank to the other.
Why teams fight for mid
Controlling mid isn't about the mere fact of standing in the zone — it's about information and timing (the moment a team decides to attack or change position). If a team holds mid, it can see where the opponents went and quickly redirect players to wherever an attack is forming. Losing mid, on the other hand, often means the opponent can safely rotate (move between sites) and enter from an unexpected direction.
On maps where mid is open and can be shot through along its entire length, an AWP sniper is often placed there — holding the distance and preventing opponents from crossing the center without losses. On maps with a narrower, more closed-off mid, a support player with grenades and an entry fragger tend to work together, pushing through a smoke or a flash.
Mid in different round situations
At the start of the round, the fight for mid often determines who will dictate the pace: the team that gains control can afford an aggressive rush on one of the sites, knowing that the flank from the middle is closed off. If mid is lost, the team has to play more cautiously and spend more utility (grenades) to cut off that area with smoke.
Closer to the middle and end of the round, mid turns into a corridor for rotations: players can run through it from one site to the other if the attack goes in an unexpected direction. That's why commentators often say "the team lost mid" — this isn't about a specific player's death, but about losing control over an entire part of the map, which affects the rest of the round.
The term is used primarily in relation to CS2 and similar tactical shooters, where the map is divided into distinct zones each with its own combat logic.