Jumping

Jumping is a mechanic that elevates the player $1 1/4$ block into the air. It also has other effects when combined with another forms of movement.

Activation
Jumping is done by pressing the key (which defaults to the space bar on PC) or the jump button on other devices (for example the 'A' button on Xbox). If auto-jump is on the player jumps whenever they brush up against the edge of a block.

Usage
Jumping normally lets the player climb up 1$1/4$ block. Players with Jump Boost I are able to jump 1$15/16$ block, and with Jump Boost II they are able to jump 2$1/2$ blocks. The effect also increases more with more levels of Jump Boost.
 * Jump height changes

Honey blocks make players jump $3/16$ of a block from them, meaning that they get an 85% jumping reduction.

Cobwebs and sweet berry bushes prevent players from jumping from them.
 * Cancellation

Sneaking and crawling, when activated by suffocation prevention $$, prevent players from jumping due to the reduced space. These two actions normally do not prevent players from jumping.

Jumping can be combined with sprinting to increase the player's movement speed. A single jump while sprinting costs four times as much hunger as a normal jump.
 * Combinations

Mobs
Most land-based mobs are able to jump when they are walking up blocks. However, they do not jump spontaneously, except in these cases:
 * Illagers and witches when celebrating a successful raid
 * Foxes, when attacking
 * Spiders and cave spiders, when attacking
 * Rabbits, as part of their normal movement
 * The killer bunny attacks similarly to spiders
 * Slimes and magma cubes, as a part of their normal movement

Horses and their variants, iron golems and endermen are able to walk up full-sized blocks without jumping.

Flying mobs (ghasts, phantoms, etc.) and water mobs (except turtles) do not jump.
 * Fishes, guardians and elder guardians flop around when out of water; they do not jump.

Trivia

 * Jumping was one of the first transportation mechanics added.
 * The longest possible distance a player can jump without effects is over 9 blocks, by sprint-jumping out of a 2-block-high ice corridor.