= impossibility, n. rare in singular.

With an and plural: An instance of this; an impossible thing; that which cannot be.

An inability to do a thing; an impossibility; (also) a statement that something cannot be done.

An impossibility. Obsolete.

With definite article: that which is or seems impossible.

blue dahlia: figurative something impossible or unattainable (no blue variety of the dahlia having been produced by cultivation).