From The Collaborative International Dictionary of English v.0.48 [gcide]:
Nonesuch \None"such`\, n.
A person or thing of a sort that there is no other such;
something extraordinary; a thing that has not its equal. It
is given as a name to various objects, as to a choice variety
of apple, a species of medic (Medicago lupulina), a variety
of pottery clay, etc.
[1913 Webster] Nonet
From WordNet (r) 3.0 (2006) [wn]:
nonesuch
n 1: model of excellence or perfection of a kind; one having no
equal [syn: ideal, paragon, nonpareil, saint,
apotheosis, nonesuch, nonsuch]