se·ries 
(sîrēz)
(sîrēz)n. pl. series
Idiom: 1. A number of objects or events arranged or coming one after the other in succession.
2. A set of stamps, coins, or currency issued in a particular period.
3. Physics & Chemistry A group of objects related by linearly varying successive differences in form or configuration: a radioactive decay series; the paraffin alkane series.
4. Mathematics The sum of a sequentially ordered finite or infinite set of terms.
5. Geology A group of rock formations closely related in time of origin and distinct as a group from other formations.
6. Grammar A succession of coordinate elements in a sentence.
7.
a. A succession of publications that present an extended narrative, such as a comic book series, or that have similar subjects or similar formats, such as a series of cookbooks.
b. A succession of individual programs presented as parts of a unified whole, such as the set of episodes of a television show or a podcast.
8.
a. Sports A number of games played by the same two teams, often in succession.
b. Baseball The World Series.
9. Linguistics A set of vowels or diphthongs related by ablaut, as in sing, sang, sung, and song.
in series
In an arrangement that forms a series.
[Latin seriēs, from serere, to join; see ser-2 in the Appendix of Indo-European roots.]
Synonyms: series, chain, progression, sequence, string, succession
These nouns denote a number of things placed or occurring one after the other: a series of days, a series of facts; a chain of command, a chain of proof; a progression of courses toward a degree, a progression of prime numbers; a sequence of a chemical reactions, the sequence of events leading to the accident; a string of islands, a string of questions; a succession of failures, a succession of actors auditioning for the play.
These nouns denote a number of things placed or occurring one after the other: a series of days, a series of facts; a chain of command, a chain of proof; a progression of courses toward a degree, a progression of prime numbers; a sequence of a chemical reactions, the sequence of events leading to the accident; a string of islands, a string of questions; a succession of failures, a succession of actors auditioning for the play.
Usage Note: Series is both a singular and a plural form. When it has the singular sense of "one set," it takes a singular verb, even when series is followed by of and a plural noun: A series of lectures is scheduled. When it has the plural sense of "two or more sets," it takes a plural verb: Two series of lectures are scheduled: one for experts and one for laypeople.