Final answer:
Validity refers to the accuracy of a given instrument or tool in measuring what it is designed to measure. Different types of validity, such as ecological validity and construct validity, can express the extent of validity. Researchers aim for both reliability and validity in their instruments.
Step-by-step explanation:
The best definition of validity is the extent to which a given instrument or tool accurately measures what it's supposed to measure. Validity can be expressed in different ways, such as ecological validity, construct validity, and face validity. While any valid measure is reliable, the reverse is not necessarily true. Researchers strive to use instruments that are both highly reliable and valid.