What exactly is mastery learning?

If I reflect on this year we all seem to have spent a large amount of it trying to understand what actually is meant by various terms and the difference between them; mastery, depth of learning, greater depth, maths mastery, secure, working towards, assessment without levels...... unsurprisingly, individuals and companies have all had a go at unpicking these terms with varying degrees of success and accuracy. It is not surprising there is confusion across schools. 

The 'Final report of the Commission on Assessment without Levels', published in September, defines mastery and clarifies it is not about working above and beyond but simply 'mastering' a skill or having a secure knowledge of a concept. Could we then argue that if a child is 'secure' with a skill they have therefore 'mastered' it?