Tick

'Tick' is a term in minecraft refering to time units.

There are 2 types of ticks, game ticks, and redstone ticks.

There are 20 game ticks per second.

There are 10 redstone ticks per second.

Game ticks messure time, and update speed for blocks. (sand takes 15 ticks to update and fall), while redstone ticks matter only for redstone updates. Repeators add 'tick delays' by 8's, meaning 1 delay is 2/5ths of a second, 2 delays is 1/8th of a second, and so on. Redstone torches passing current (in inverters per say) takes 15 RT's, or 1 1/2 seconds.

For more information, go to the official Minecraft Wiki: Ticks