Jumping is not present in vanilla Doom and Heretic, but implemented in Hexen and Strife. Jumping can be selectively enabled or disabled through MAPINFO map definitions and through the gameplay options menu. The console variables sv_allowjump and sv_nojump govern whether jumping is possible, possible if not disallowed by the map, or impossible.
Jumping works by giving the player an upward vertical velocity amount equals to their Player.JumpZ value. Each tic, the player will be moved upward by that velocity, then the velocity is decreased by the gravity factor (by default, 1). The player will keep raising as long as their upward velocity isn't decreased to 0 or less by gravity, and as long as they do not collide with an obstacle, such as the ceiling or a flying enemy. Jumping does not prevent climbing up to the player's MaxStepHeight (24 by default for most games, except Strife where it is 16), so a jump height of 36 and a step height of 24 mean that a player can climb over an obstacle up to 60 units tall.
On the following table, the default gravity and the default player jump speed for (9 for Hexen, 8 for the other games) are highlighted. Other values are provided for illustration.
On a flat area (landing sector has the same height as jumping sector), with constant gravity, and if the jump isn't obstructed by collision with an obstacle, the player's landing speed should be approximately the same as the jump speed. (Not exactly the same, because if the player reaches a vertical velocity of exactly 0 at the jump's apex, which may not happen depending on the gravity factor, the downward velocity from gravity will be doubled for that tic.) This means that a high jumping speed can potentially result in falling damage. This can also happen if during the jump the player moves in a sector with a lower floor height, or a stronger gravity factor.