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.

Jump height changes
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.

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

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

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.

Combinations
Jumping can be combined with sprinting to increase the player's movement speed. This also reduces the hunger bar faster.

Other mobs
Certain mobs are able to jump:
 * Villagers (but they prefer not to jump)
 * Illagers (when celebrating a successful raid)
 * Witches (when celebrating a successful raid)
 * Spiders and cave spiders jump when they attack.
 * Rabbits jump as part of their movement.
 * The Killer Bunny attacks similarly to Spiders.
 * Most other mobs.
 * This is only the case if the player is luring the mob, or if the mob is hostile. If the mob flies, or is stationary, or is a water mob (except guardians, elder guardians and turtles), it does not jump.
 * Fishes flop around, not jump.

Trivia

 * Jumping was one of the first transportation mechanics added.
 * The highest possible distance a player can jump without effects is over 9 blocks, using ice momentum under a 2 block ceiling.
 * Using only headhitters and no ice, the player can jump slightly over 5 blocks.
 * In 1.8.x versions, the player is able to jump 5 blocks using elevation momentum, which has a similar effect to headhitter momentum by creating less air time and therefore allowing more speed.
 * Using only "normal" blocks, no ceiling, and flat momentum, the player can jump a little under 5 blocks with maximum momentum.
 * However, using a combination of unusual mechanics (such as damage boosting, honey blocks, etc.) in specific versions, the player can jump even further than 10 blocks.