In the context of assessment, validity is the degree to which an assessment measures that which it intends to measure. For example, a valid assessment of maths would measure how well an individual has understood say fractions or decimals, as opposed to how well they can read the words in a long maths word problem. 

It is considered good practice when using the term validity to follow them with the phrase ‘…for the purpose of X’. For instance, teachers and school leaders might talk about ‘the validity of the Year 6 maths exam for the purpose of assessing pupils’ progress in maths during Year 6’.