In Doom speedrunning terminology, a grab occurs when the player obtains an item from a location they are not supposed to do it from. It can occur with items placed behind impassible, transparent walls, and raised platforms with items just out of the player's reach, as well as solid walls if they are thin enough. This is usually done by straferunning against the wall in question, toward the item itself, or where it is located if it is behind a solid wall. How fast one needs to go depends on how far away the object is. If this is successful, the game will consider the item to be within reach, and will give it to the player.
Widely used in Compet-n, the most common application of this technique is obtaining otherwise unreachable keys, which can result in a massive shortcut, and is sometimes useful in other ways. Most notably, the red skull key grab on Ultimate Doom E4M1 made speedruns on the otherwise difficult map easier; no one has ever completed the level under NM speed rules without it.  It is also sometimes used to acquire other kinds of items useful for certain demo categories, such as in Ultimate Doom E4M2, where the BFG 9000 can be taken through the eastern wall of the room directly behind it, although one must run forward rather than straferun to do this. This particular grab is very useful when performing Nightmare! demos on this map, or episode runs for Thy Flesh Consumed. It has even found application in multiplayer competition, such as in Doom II MAP01, where it can be used to grab the plasma rifle on the raised platform, without having to take the lift and jump from it.
Grabs are disabled in many Source ports.