Series:

(1.) A group of separate items related to one another by the fact that each item bears, in addition to its own title proper, a collective title applying to the group as a whole. The individual items may or may not be numbered.

(2.) Each of two or more volumes of essays, lectures, articles, or other writings, similar in character and issued in sequence (e.g., Lowell's Among my books, second series).

(3.) A separately numbered sequence of volumes within a series or serial (e.g., Notes and queries, 1st series, 2nd series, etc.).