According to Lending Tree:

A 5 Year ARM is a loan with a fixed rate for the first five years. After that, it has an adjustable rate that changes once each year for the remaining life of the loan. Because the interest rate can change after the first five years, the monthly payment may also change.

A 5 year ARM, also known as a 5/1 ARM, is a hybrid mortgage.  A hybrid mortgage combines features from an adjustable rate mortgage (ARM) and a fixed mortgage. It begins with a fixed rate for a specified number of years, but then changes to an ARM with the rate changing every year for the rest of the term of the loan. With a 5 year ARM, the interest rate is fixed for a period of five years, after which it will be adjusted annually.