Why is the ninth month called September?
1610s, "a number or set of things of one kind arranged in a line," from Latin series "row, chain, series, sequence, succession," from serere "to join, link, bind together, arrange, attach, put; join in speech, discuss," from PIE root *ser- (3) "to line up, join" (cf. Sanskrit sarat- "thread," Greek eirein "to fasten together in rows," Gothic sarwa (plural) "armor, arms," Old Norse sörve "necklace of stringed pearls," Old Irish sernaid "he joins together," Welsh ystret "row").
Meaning "set of printed works published consecutively" is from 1711. Meaning "set of radio or television programs with the same characters and themes" is attested from 1949. Baseball sense "set of games on consecutive days between the same teams" is from 1862.
series se·ries (sēr'ēz)
n. pl. series
A number of objects or events arranged or coming one after the other in succession.
A group of objects related by linearly varying successive differences in form or configuration, as in a radioactive decay series.