semiannual

As an adjective semiannual

is biannual: occurring twice a year.

As an adverb semiyearly is

(uncommon) semiannually.

As adjectives the difference between semiannual and triennial

is that semiannual is biannual: occurring twice a year while triennial is happening every three years.

As a noun triennial is

a third anniversary.

As adjectives the difference between semiannual and pretty

is that semiannual is biannual: occurring twice a year while pretty is cunning; clever, skilful.

As an adverb pretty is

somewhat, fairly, quite; sometimes also (by meiosis) very.

As a noun pretty is

something that is pretty.

As a verb pretty is

to make pretty; to beautify.

As a noun taxonomy

is the science or the technique used to make a classification.

As an adjective semiannual is

biannual: occurring twice a year.