What is a "rate lock period"?

A rate lock or a rate commitment is a lender's promise to hold a certain interest rate for a specified period of time while your application is processed. This prevents you from going through your whole application process and at the end of it finding out the interest rate has gone up.

A rate lock period can vary in length, and longer ones usually cost more. As an example one may pay slightly higher for an extended lock. Rate lock periods are commonly 15, 30, 45 and 60 days.