- Adjective. Optimal; being the best possibility.
- Adjective. Perfect, flawless, having no defects.
- Adjective. Existing only in the mind; conceptual, imaginary.
- Adjective. (mathematics) Not actually present, but considered as present when limits at infinity are included.
- Noun. A perfect standard of beauty, intellect etc., or a standard of excellence to aim at.
- Noun. (mathematics, order theory) A non-empty lower set (of a partially ordered set) which is closed under binary suprema (a.k.a. joins).[http://en.wikipedia.org/wiki/Boolean_prime_ideal_theorem#Prime_ideal_theorems].
- Noun. (context, for example) A subring closed under multiplication by its containing ring.