An amount of money that is regularly paid at the beginning of a term. Rent is the classic example of an annuity in advance because it is a sum of money paid at the beginning of the month to cover the 30 days to follow. An annuity in advance is also called an "annuity due."
The opposite of an annuity in advance is an annuity in arrears (also called an "ordinary annuity"). Mortgage payments are an example of an annuity in arrears. Like rent payments, mortgage payments are due on the first of the month; however, the mortgage payment covers the previous month’s interest and principal on the mortgage loan.
One instance where the difference between an annuity in advance and an annuity in arrears matters is in the valuation of income properties. If payments are received at the beginning of the rental period rather than at the end of the rental period, their present value increases.
Don't be confused by the name: an "annuity in advance" is not the type of investment product that we usually think of when we hear the word "annuity" in finance.