A bearded iris with white or tinted white standards (upper petals) and colored falls (lower petals).
Here is an example, and here is another.
Etymology: Latin, meaning beautiful, attractive, pleasant, charming, etc.
The above definition is specific to the iris community, but you will also see the word in scientific names: Passerina amoena (lazuli bunting), Caladenia amoena (charming spider orchid), Ceratosoma amoenum (clown nudibranch).
(Yes, I received an iris catalog today.)