Tick
Basics
What it means
The smallest possible price change for a specific market, and also a single traded transaction. The tick size sets the minimum step a price can move.
Example
For most US stocks the tick size is $0.01, so a price can move from $10.00 to $10.01 but not to $10.005. In ES futures, one tick is 0.25 points ($12.50).
Mistake beginners make
Beginners try to place limit orders between ticks (e.g. $10.005) and are confused when the broker rejects them; prices can only step in tick-sized increments.