A crush is a very uncommon method of being KOed, which involves the player's character being stuck between two solid platforms that move into each other; if the player is unable to move out of the way from the common collision point of the two platforms, then they will be crushed when the two platforms meet. This results in an instant KO regardless of the player's accumulated damage.
In Brawl, crushing is a specific property of the code for moving objects, meaning it does not have a hitbox and cannot be prevented through invincibility (such as the Super Star item). It is also coded to only occur in The Subspace Emissary; regular stages cannot crush by any means. In Smash 4 onwards, the effect of crushing on stages is caused by a special hitbox generated by the stage, and as such can be prevented through invincibility. No object in Stage Builder is coded to have a crushing hitbox.
Stages that can crush
On Midgar, summoning Odin via Summon Materia on the stage will cause him to slice the stage in half. After some time, the two halves of the stage will come back together; players caught in between these two halves will be crushed.
In Ultimate, on Distant Planet, when the Red Bulborb swallows a fighter, they will be crushed when its mouth closes. This did not occur in Brawl and for 3DS, where instead the fighter would be trapped inside and carried offscreen for a guaranteed KO.
In The Subspace Emissary
The Subspace Emissary features several stages with crushes in them; owing to the mode's focus on platforming, the player is tasked with correctly timing their movements as to avoid these crushes. The following stages have crushes: