based on custom rather than documentation; "an unwritten law"; "rites...so ancient that they well might have had their unwritten origins in Aurignacian times"- J.L.T.C.Spence
a base hit on which the batter stops safely at first base
existing alone or consisting of one entity or part or aspect or individual; "upon the hill stood a single tower"; "had but a single thought which was to escape"; "a single survivor"; "a single serving"; "a single lens"; "a single thickness"
used of flowers having usually only one row or whorl of petals; "single chrysanthemums resemble daisies and may have more than one row of petals"