The phrase "once-a-year" refers to something that occurs only once in a year. Synonyms for this phrase include "annual," "yearly," "once per annum," "once yearly," and "per annum." "Annual" is the most commonly used synonym for "once-a-year," and it can refer to anything that happens on a recurring basis every year. "Yearly" also means the same thing and is often used interchangeably with the word "annual." "Once per annum" means something that happens once during a year, and "once yearly" means the same thing in plain English. "Per annum" refers to the yearly rate of anything. All of these synonyms indicate that something happens only once in a year.