Definitions:
only one; not one of several

unmarried or not involved in a stable sexual relationship

consisting of one part

free from duplicity or deceit; ingenuous

an individual person or thing rather than part of a pair or a group.

a hit for one run.

(especially in tennis and badminton) a game or competition for individual players, not pairs or teams.

a system of change-ringing in which one pair of bells changes places at each round.

choose someone or something from a group for special treatment

thin out (seedlings or saplings)

reduce (a railway track) to a single line

hit a single