(not-comparable) Not accompanied by anything else; one in number.
(not-comparable) Not divided in parts.
(not-comparable) Designed for the use of only one.
(not-comparable) Performed by one person, or one on each side.
(not-comparable) Not married, and (in modern times) not dating or without a significant other.
(not-comparable) Having only one rank or row of petals.
(not-comparable, obsolete) Simple and honest; sincere, without deceit.
(not-comparable) Uncompounded; pure; unmixed.
(not-comparable, obsolete) Simple; foolish; weak; silly.

A 45 RPM vinyl record with one song on side A and one on side B.
A popular song released and sold (on any format) nominally on its own though usually having at least one extra track.
One who is not married or does not have a romantic partner.
A score of one run.
A hit in baseball where the batter advances to first base.
A tile that has a different value (i.e. number of pips) at each end.
(US, informal) A bill valued at $1.
(UK) A one-way ticket.
(Canadian) A score of one point, awarded when a kicked ball is dead within the non-kicking team's end zone or has exited that end zone.
(in-plural) A game with one player on each side, as in tennis.
One of the reeled filaments of silk, twisted without doubling to give them firmness.
(Scotland, UK, dialectal) A handful of gleaned grain.
A floating-point number having half the precision of a double-precision value.
A shot of only one character.
A single cigarette.
(obsolete) Synonym of single-driver.