Final answer:
A simple definition of reliability is the consistent reproducibility of an instrument's results under the same conditions.
Step-by-step explanation:
A simple definition of reliability would be the reproducibility of an instrument. In other words, reliability is the consistency of a measure; a measure is deemed reliable when it yields the same results under the same circumstances. It's a crucial component in research to ensure that if a study is reproduced, the results are likely to be replicated. However, reliability alone doesn't mean an instrument is valid, as validity refers to whether an instrument accurately measures what it's supposed to measure. But for any measure to be valid, it must first be reliable, indicating that the results are reproducible and consistent whenever the measurement is taken.