Single, sole, unique, lone, solitary, separate, particular can all mean one as distinguished from two or more or all others.
Something single is not accompanied or supported by, or combined or united with, another.
Something sole is the only one that exists, that acts, that has power or relevance, or that is to be or should be considered.
Something unique (see also STRANGE ) may be the only one of its kind in existence or it may stand alone because of its unusual character.
Something lone (see also ALONE ) is not only single but also separated or isolated from others of its kind; the word often replaces single in technical or poetic context.
Something solitary (see also ALONE ) stands by itself, either as the sole instance or as a unique thing.
Something separate (see also DISTINCT 1 ) is not only single, but disconnected from or unconnected with any of the others in question.