Definition of 'Tick'
The minimum upward or downward movement in the price of a security. The term "tick" also refers to the change in the price of a security from trade to trade. Since 2001, with the advent of decimalization, the minimum tick size for stocks trading above $1 is 1 cent.
Investopedia explains 'Tick'
Prior to April, 2001, the minimum tick size was 1/16th of a dollar, which meant that a stock could only move in increments of $0.0625. Needless to say, while the introduction of decimalization has benefited investors through much better bid-ask spreads and better price discovery, it has also made market-making a less profitable (and riskier) activity.
The term "tick" is also used in reference to the direction of the price of a stock, with an "uptick" referring to a trade where the transaction has occurred at a price higher than the previous transaction, and a "downtick" referring to a transaction that has occurred at a lower price. In this context, the uptick rule refers to a trading restriction that prohibits short selling, except on an uptick, presumably to alleviate downward pressure on a stock when it is already declining.