annually vs per annum

annually and per annum both are adverbs.

Word	Noun	Adjective	Verb	Adverb
annually	No	No	No	Yes
per annum	No	No	No	Yes

As adverbs, annually and per annum are synonyms defined as:

annually and per annum: by the year; every year (usually with reference to a sum of money paid or received)

Other synonyms of annually include each year, p.a., per year.

annually (adverb)	per annum (adverb)
without missing a year	by the year; every year (usually with reference to a sum of money paid or received)
by the year; every year (usually with reference to a sum of money paid or received)