Per annum means once per year.

How long is an annum?

Per annum means yearly or annually.

What is difference between year and annum?

Key Difference: Year is the time taken by a planet to complete one revolution around the sun. Annum also denotes a period of year but is derived from Latin word 'annus'; the word annum is mostly used in terms of finance like wages or salary. Year is the time taken by a planet to complete one revolution around the sun.