A period (.) was a punctuation mark used to show the end of a sentence or separate letters in an abbreviation.
In 2371, Kathryn Janeway read the name tag affixed to the occupant of a cryostasis chamber, which said "A period E-A-R-H-A-R-T" and deduced (correctly) that it was Amelia Earhart. (VOY: "The 37's")