Test-retest reliability is a measure used to assess the consistency of a test over time. It involves administering the same test to the same group of people on two different occasions. If the results are similar, the test is considered reliable, indicating that it produces stable and consistent outcomes. This type of reliability is important in fields like psychology and education, where tests are used to evaluate abilities or traits. High test-retest reliability suggests that the test is effective in measuring what it intends to measure, making it a valuable tool for researchers and practitioners.