Transaction Volume
The total value moved over a period, either settled on a blockchain or traded on exchanges, depending on which volume is meant.
Two different measurements share this name and are routinely confused. On-chain transfer volume sums the value moved by transactions, and raw figures are inflated by change returning to the sender on UTXO chains, by exchanges shuffling their own wallets, and by automated contract activity; adjusted versions try to strip these out using heuristics. Exchange trading volume sums executed trades on venues, and is affected by wash trading, incentive programs, and the question of which venues a provider trusts enough to include. Both are denominated in an asset and converted to a reporting currency at some rate, so the series carries price movement inside it. Always check which volume a chart shows and whether it is adjusted.
In practice
An exchange consolidating funds between its own hot and cold wallets can add a large amount to raw on-chain volume without any user activity.
The common misunderstanding
Raw on-chain volume is not economic payment activity: a large share of it is change outputs, internal exchange transfers, and automated movements.