Ticks
Ticks are the standard unit of time in Minecraft, with one tick being equal to 50 milliseconds.
Tickrate
The physics engine runs at 20 ticks per second, meaning the game's physics are updated every 50ms.
This includes the player’s position and speed, the environment, and entities.
In-game actions are performed at the end of each tick, regardless of the time or order in which they were called.
Due to the game working this way, inputs may take up to 50ms of delay between the button press and their activation.
Turn Tick
Mouse Movement is not inherently tied to tickrate, but rather the framerate.
However, the game has to keep a copy of the player's rotation for movement calculations, which it updates once every tick. The moment the player's rotation is copied is called the turn tick.
From the player's perspective, there is no way to control when the turn tick happens. This has a severe impact on turn-based jumps, which become partially luck-based as a result: given the same (smooth) turning sequence, the player's resulting movement could be quite different.
One solution is to turn instantly every 50ms to land precisely on the wanted angle. This is feasible for simple turn strats such as 45° Strafes, but not for more complex strats that would require smooth turning.