early 14c., "body of individuals born about the same period" (historically 30 years but in other uses as few as 17), on the notion of "descendants at the same stage in the line of descent," from Old French generacion "race, people, species; progeny, offspring; act of procreating" (12c., Modern French génération) and directly from Latin generationem (nominative generatio) "generating, generation," noun of action from past-participle stem of generare "bring forth, beget, produce," from genus "race, kind" (from PIE root *gene- "give birth, beget," with derivatives referring to procreation and familial and tribal groups).
From late 14c. as "act or process of procreation; process of being formed; state of being procreated; reproduction; sexual intercourse;" also "that which is produced, fruit, crop; children; descendants, offspring of the same parent."
Generation gap is recorded by 1967; generation x for the (American) generation born after Baby Boomers (c. 1965 - c. 1979) is from 1991, by author Douglas Coupland (b.1961) in the book of that name; abbreviation gen X is by 1997; generation y is attested by 1994 but did not catch on. Adjectival phrase first-generation, second-generation, etc. with reference to U.S. immigrant families is from 1896. Related: Generational.