"afterward," 16c., comparative of late. A modern formation; the Old English comparative lator developed into latter. As a salutation, "farewell," from 1954, U.S. colloquial, short for adverbial use in (I'll) see you later.
he's going to the store but he'll be back here later
I'll see you later
he stayed later than you did
a later symptom of the disease
later medical science could have saved the child