In modern use the more common word is later, which is from mid-15c. and is perhaps a new formation or a variant of this word. Latter survives mostly in the phrase the latter, which, with the former is used to avoid repetition (but sometimes incorrectly, when more than two are involved).
1530s, "later in time," from Latin posterior "after, later, behind," comparative of posterus "coming after, subsequent," from post "after" (see post-). Meaning "situated behind, later in position than another or others" is from 1630s. Related: Posterial.
late 14c., posteriorite, "condition of occurring later in time, state of being subsequent," from Old French posteriorite (Modern French postériorité), from Medieval Latin posterioritatem (nominative posterioritas), from Latin posterior "later" (see posterior (adj.)).