A tick, in trading, has two related meanings. The first is the smallest step by which the price of an instrument can change on a given market, formally its tick size. Prices are quoted only in multiples of this increment, which is set by the exchange. In futures the money value of one tick is fixed by the contract: the E-mini S&P 500 future, for example, moves in steps of 0.25 index points, each worth $12.50 per contract.
The second meaning is a single event in a stream of market data: one trade, or one change to the best bid or ask. Data recorded at this level is tick-by-tick data, usually stored in a tick database. A trade above the previous price is called an uptick and one below it a downtick. The last traded price is the most recent tick.