A warp is a bug that occur in the Build engine when the player is allowed to teleport from a location to another, and it became popular as a deathmatch trick. The Plutonium Pack expansion pack of Duke Nukem 3D often fix various possible warps during the first three episodes.
A map created in Build engine is not constructed in a truly 3D base, it uses a simply map that has different floor and ceiling z every sector. Different from DooM, the game allow a sector to cross another, which allow different sections of a map exist at the same space. For example: in Hollywood Holocaust the cinema exit corridor exist at the same space as the red key card area; A heavy usage of this is present on map Tier Drops.
A warp occur when the player position is verified as a valid location, for example outside a sector or in a sector which the z of the ceiling equal to the floor (I.e. inside a column or smashed), and them there "is" a valid location that is the other sector crossing the same place. Basically the game will place the player in the nearest position over that x and y coordinate that has a valid location. It would be easy in early (1.3d) version of Hollywood Holocaust to go to red key card area if the player smashed himself using the rative door in the exit corridor from the cinema.