Latter means the second of two things just mentioned. First one, then the other, and this is the other.
It pairs with former, and the two only work when exactly two things have been named, which is the rule people most often break. It always takes the. There is a separate sense meaning later, which survives in the latter half of the century and in latter-day, meaning modern. The word is not the comparative of late in modern English, though it once was, and later has taken that job entirely.
1adjectivelatterThe second of two things mentioned.
“Tea or coffee were offered, and he chose the latter.”
2adjectivebelongingBelonging to the later part of a period.
“It was built in the latter half of the century.”
Too few real examples of this word to estimate which sense is commonest.
Latter is the old comparative of late, which makes it a doublet of later: the same word twice over, one kept for order and one for time.
Old English
lætra
Middle English
latter
English, today
latter
Built from
From Middle English latter, lattere, lattre, from Old English lætra, comparative form of læt (“late”). Equivalent to late + -er, thus a a doublet of later; also, related to last, whose doublet is latest.
Etymology from Wiktionary, CC BY-SA