A book that contains two or more individual issues of a series, often a trade (TPB or trade paperback - though this can also refer to a hardcover edition). A trade is generally 6 issues of a series printed together and referred to as a volume.
For example, Daredevil was up to Volume 3 in 2011, but Marvel released the first six issues as Daredevil Volume 1, and the next 6 as Volume 2. They then reprinted the same series in hardback of 10 issues, which were also termed Volume 1.
As opposed to the series volume, which is the ongoing story. A change in series volume can refer to a reboot or relaunch of the series (e.g. Marvel published Daredevil consecutively from 1964 through to 1998 without renumbering or rebooting. Issues #1-#380 printed over this time are Volume 1 in the series. Volume 2 of Daredevil started in 1998, after the series was cancelled and started over.)