singular

from The American Heritage® Dictionary of the English Language, 5th Edition.

• adjective Being only one; individual; lone.
• adjective Being the only one of a kind; unique.
• adjective Being beyond what is ordinary, especially in being exceptionally good; remarkable.
• adjective Strange or unusual.
• adjective Of, relating to, or being a noun, pronoun, or adjective denoting a single person or thing or several entities considered as a single unit.
• adjective Of, relating to, or being a verb expressing the action or state of a single subject.
• adjective Logic Of or relating to the specific as distinguished from the general; individual.
• noun The singular number or a form designating it.
• noun A word having a singular number.

from The Century Dictionary.

• Being a unit, or one only; single.
• Separate or apart from others; alone.
• Pertaining to solitude, or separation from others; concerned with or involving solitude.
• Pertaining to one person or thing; individual; also, pertaining to individual persons or things; in logic, not general; being only in one place at one time.
• In grammar, denoting or relating to one person or thing: as, the singular number: opposed to dual and plural. Abbreviated singular
• Having no duplicate or parallel; unmatched; unexampled; unique; being the only one of its kind.
• Out of the usual course; unusual; uncommon; somewhat strange; a little extraordinary: as, a singular phenomenon.
• Hence Of more than average value, worth, importance, or eminence; remarkable; fine; choice; precious; highly esteemed.
• Not complying with common usage or expectation; hence, eccentric; peculiar; odd: as, he was very singular in his behavior.
• In mathematics, exceptional.
• Synonyms and Unwonted, exceptional, unparalleled.
• Strange, Odd, etc. See eccentric.
• noun That which is singular, in any sense of the word; that which is alone, separate, individual, unique, rare, or peculiar. See singular, a.
• noun In grammar, the singular number.
• noun In hunting, a company or pack: said of boars.
• noun In logic, that which is not general, but has real reactions with other things.

from the GNU version of the Collaborative International Dictionary of English.

• noun obsolete An individual instance; a particular.
• noun (Gram) The singular number, or the number denoting one person or thing; a word in the singular number.
• adjective obsolete Separate or apart from others; single; distinct.
• adjective obsolete Engaged in by only one on a side; single.
• adjective (Logic) Existing by itself; single; individual.
• adjective (Law) Each; individual.
• adjective (Gram.) Denoting one person or thing; ; -- opposed to dual and plural.
• adjective Standing by itself; out of the ordinary course; unusual; uncommon; strange.
• adjective Distinguished as existing in a very high degree; rarely equaled; eminent; extraordinary; exceptional.
• adjective Departing from general usage or expectations; odd; whimsical; -- often implying disapproval or censure.
• adjective Being alone; belonging to, or being, that of which there is but one; unique.
• adjective (Math.) a point at which the curve possesses some peculiar properties not possessed by other points of the curve, as a cusp point, or a multiple point.
• adjective (Logic) a proposition having as its subject a singular term, or a common term limited to an individual by means of a singular sign.
• adjective (Civil Law) division among individual successors, as distinguished from universal succession, by which an estate descended in intestacy to the heirs in mass.
• adjective (Logic) a term which represents or stands for a single individual.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• adjective Being only one of a larger population.
• adjective Being the only one of the kind; unique.
• adjective Distinguished by superiority, coming across as such.
• adjective Being out of the ordinary, coming across as such.
• adjective grammar Referring to only one thing or person.
• adjective linear algebra, of matrix Having no inverse.
• adjective linear algebra, of transformation Having the property that the matrix of coefficients of the new variables has a determinant equal to zero.
• noun grammar A form of a word that refers to only one person or thing.