Tick

The atomic unit of market data. Every chart, every indicator and every candle is an aggregation of ticks, and a tick chart plots them without aggregation at all, so its horizontal axis measures activity rather than time: a quiet hour occupies less width than a busy minute. The word also carries a second, older sense of the size of a single price step, which is the sense meant in the phrase tick size, and the two meanings sit side by side in ordinary use.

What a tick represents depends on the market it came from. On an exchange, a tick usually accompanies a transaction, so the tick record is close to a record of trades. In foreign exchange, which is dealt over the counter, a tick is a new quote from a price source, and a quote can change without a single unit being traded. Counting ticks in that market therefore counts quoting activity, which correlates with trading but is not the same measurement.

Two things are commonly missed. There is no single true tick stream for a decentralised market: two providers aggregating different sources produce different tick counts and slightly different extremes for the same instrument and the same second, and neither is wrong. And tick history is expensive to store, so platforms frequently retain full ticks for a short recent window and bars for everything older, which is why a study run on tick data and a study run on bars can disagree about the same period.