"Mastery" means having expert knowledge, skill, or control over something. It implies being highly proficient and skilled in a particular subject or activity. Full definition
The report has a very clear definition of mastery in learning, which it has laid out in response to the fact that this term is currently being used in many different ways. (edisonlearning.net)
I've also been able to design assessments that allow for multiple methods of demonstrating mastery of content standards. (edutopia.org)
In its simplest form, classroom assessment is about collecting data, looking for mastery of content, and guiding instruction. (thoughtco.com)