In the context of traditional financial market, trading volume refers to the sales volume of financial products (number of shares) traded in a given period of time. In the context of cryptocurrency, trading volume means the total number of coins or tokens traded within a specific time window. Here, the value of the traded coins or tokens is often stated in USD.
For example, Bitcoin’s trading volume in the last 24 hours is $34,540,612,954.
See also: Circulating Supply.