In a general sense, a series is a related set of things that occur one after the other (in a succession) or are otherwise connected one after the other (in a sequence). The specific meaning depends on the context.
Sciences
Entertainment
2 in a series = Sequel or Duology
3 in a series = Trilogy or Triptych
4 in a series = Tetralogy
5 in a series = Pentalogy
6 in a series = Sextology (sometimes hexology)
7 in a series = heptology
8 in a series = octology
9 in a series = nonology
10 in a series = decology
11 in a series = undecology
12 in a series = dudecology
Other
- On U.S. currency, series refers to the year appearing on the front of a bill, indicating when the bill's design was adopted.