‘Something that can be measured and used as a standard that other things can be compared with’ (Oxford Learner’s Dictionaries).
‘A systematic process of comparing the activities, processes and/or performance of a programme, organization, country, learner, etc. against a theoretical, political or existing reference with the aim of identifying ways to improve performance’ (CEDEFOP (2011) in UNESCO-IBE, Glossary of Curriculum Terminology).