Latin phrase, meaning "every year" or "yearly", normally used in sentences such as "expense per annum".
Example: "This year, I am proposing to widen the standard rate tax band for a single person by £3,000 from £14,000 to £17,000 per annum." (Charlie McCreevy, Ireland Minister of Finance in his Dec 1 1999 Financial Statement Speech).
In the specialized context of catholic liturgy, per annum refers to the Sundays that aren't in any special time of the year, that's to say the 34 Sundays that do not fall in Advent, Easter, Lent...