A glide refers to one of several related Doom movement tricks used in speedrunning that result in unusual or unexpected player motion, usually leading to skipping past an obstacle such as a barred door, line trigger, or narrow space intended to be entirely impassible. Tricks in this class are a chief means of sequence breaking in Doom engine games.
A squeeze glide, often referred to simply as a glide or a "bar glide," is the most common of the glide tricks. It involves the player fitting into a space of exactly 32 units wide with the goal of skipping past a set of bars or other obstacle to player progression. Because the player is also 32 units wide, this means that the player must align themselves perfectly with the gap in question before moving through, down to a precision of 1/65536 of a unit. The quantized and somewhat inaccurate nature of the movement code can make this a difficult task.
This trick was discovered in 1999 by Jonathan Rimmer on MAP16: Suburbs (Doom II), and subsequently recorded in his 00:23 UV speed demo of the level (lv16-023.zip). In this demo, it is used to squeeze past the bars blocking the exit, skipping the red key in the process.
An easy demonstration of a squeeze glide can be seen in accessing the secret exit of MAP31: Degravitation (Interception); after raising two "guard rails" 32 units apart the player can access a teleporter to a spot exactly between them, and run forward through a gap in two walls exactly 32 units wide.
Guided / guideless glides
Speedrunners have developed more specific terminology for these types of glides since they are by far the most common. A distinction is often drawn between guided glides, where there is a wall abutting one side of the 32-unit gap, making it easier to line up in front of it correctly, and guideless glides, where the gap is "free-floating" and the player must attempt to align with it purely by sight.
A skip glide, also sometimes referred to as a "bar glide," is a rarer trick, best known in its use in speedruns of Doom II MAP21: Nirvana. The Doom movement code checks to see if a player's desired new position is valid, and if so, it will move them to that position without checking if there is anything in between the old and new positions that logically ought to block movement. In a skip glide, the player moves through a diagonal gap that is smaller than their own diagonal width by moving from a valid position just before collision with the gap, to a valid position just on the other side of the diagonal gap.
A wobble glide is a variety of skip glide where the player builds up momentum while standing still in order to pass through a narrow gap. It is otherwise effectively identical to the standard skip glide, but is generally used where there is not enough room to gain momentum through running normally. Wobble glides are usually achieved by running against impassible 2-sided linedefs or things, which generally preserve a player's momentum, and then sliding off the edge and through the gap; on some occasions they can be performed against a normal wall, however, using the same momentum preservation trick as in the void glide. It is so named because the momentum also results in the player's weapon visibly "wobbling" back and forth during the setup and execution of the trick.
A void glide is an extremely rare trick where the player gains enough momentum to pass entirely through a solid wall. This usually results in exiting to the "void" outside of the boundaries of a Doom map. Generally speaking, when a player runs into a wall, their momentum is redirected in the direction the wall is traveling and clipped. But with precise positioning and angle, a player can run into a corner in such a way that their momentum is not only preserved, but actually amplified; when kept up for several tics in a row, this can result in the player's momentum increasing to over 32 units/tic, at which point the player effectively performs a "wobble glide" that passes the player entirely through the solid wall.