Volume

The quantity traded during an interval, reported per bar on a chart and per session in market statistics. On an exchange the figure is a real count taken from the matching engine, with each contract counted once however many times it later changes hands, and it is a measure of activity in the interval rather than of positions in existence. That second quantity is open interest, and the two answer different questions: a busy session in which positions are opened and closed again shows heavy volume and unchanged open interest.

Spot foreign exchange has no equivalent figure, because the market is dealt over the counter across many venues with no consolidated record of what traded. What charting platforms display for such an instrument is tick volume: a count of how many times the quote changed inside the bar. It measures how busy the feed was, not how much was dealt, and each provider sees only the flow reaching its own price feed, so two platforms report different tick volume for the same pair over the same minute.

Comparison is the usual trip, and it fails in three places. A figure means nothing without the average it is being read against, since the same count is heavy for one hour of the day and thin for another. Volume in a futures market splits across delivery months as a roll approaches, so the front month's figure falls for reasons that have nothing to do with interest in the market. And how well tick counts stand in for traded size is an open question: published work reports a close relationship on heavily traded pairs in ordinary conditions, and weaker agreement in fast markets, which is where the substitution is most often relied on.