adj Not accompanied by anything else; one in number.
adj Not divided in parts.
adj Designed for the use of only one.
adj Performed by one person, or one on each side.
adj Not married or (in modern times) not involved in a romantic relationship without being married or not dating anyone.
adj (botany) Having only one rank or row of petals.
adj (obsolete) Simple and honest; sincere, without deceit.
adj Uncompounded; pure; unmixed.
adj (obsolete) Simple; foolish; weak; silly.
n (music) A 45 RPM vinyl record with one song on side A and one on side B.
n (music) A popular song released and sold (on any format) nominally on its own though usually having at least one extra track.
n One who is not married or does not have a romantic partner.
n (cricket) A score of one run.
n (baseball) A hit in baseball where the batter advances to first base.
n (dominoes) A tile that has a different value (i.e. number of pips) at each end.
n (US, informal) A bill valued at $1.
n (UK) A one-way ticket.
n (Canadian football) 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.
n (tennis, chiefly in the plural) A game with one player on each side, as in tennis.
n One of the reeled filaments of silk, twisted without doubling to give them firmness.
n (UK, Scotland, dialect) A handful of gleaned grain.
n (computing, programming) A floating-point number having half the precision of a double-precision value.
n (film) A shot of only one character.
n A single cigarette.
n (rail transport, obsolete) Synonym of single-driver.
v To identify or select one member of a group from the others; generally used with out, either to single out or to single (something) out.
v (baseball) To get a hit that advances the batter exactly one base.
v (agriculture) To thin out.
v (of a horse) To take the irregular gait called singlefoot.
v (intransitive, archaic) To sequester; to withdraw; to retire.
v (intransitive, archaic) To take alone, or one by one; to single out.
v (transitive) To reduce (a railway) to single track.