Definition 1: The ability of a feedback instrument to measure what it was intended to measure; also, the degree to which inferences derived from measurements are meaningful
Source: Glossary, American Society for Quality (ASQ)
Definition 2: The extent to which methodologies and instruments measure what they are supposed to measure. A data collection method is reliable and valid to the extent that it produces the same results repeatedly. Valid evaluations are ones that take into account all relevant factors, given the whole context of the evaluation, and weigh them appropriately in the process of formulating conclusions and recommendations.
Source: UNFPA(2008). Tool Number 1, Programme Managers’ Planning, Monitoring and Evaluation Toolkit.