【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.

Something  【particular】  (see also SPECIAL ) ( CIRCUMSTANTIAL ) ( NICE 1 ) is the 【single】 or numerically distinct instance, member, or example of the whole or the class considered or under consideration.