My own take on them is that proficiency is a term from SLA (second language acquisition) and refers to what a student can do with L2, Latin in this case. You should be able to get the ACTFL proficiency guidelines for Latin. If not, let me know and I’ll e-mail them to you (bad copies) or fax them to you. They’re in the great big bible, Standards for Foreign Language Learning in the 21st Century from ACTFL.
Proficiency is defined in Shrum & Glisan’s The Teacher’s Handbook p. 216 thus: “P. is the ability to use language to perform global tasks or language functions within a variety of contexts/content aresas, with a given degree of accuracy, and by means of specific text types.”
(I can explicate that if the terminology is not familiar to you)
Mastery is an educational term referring to learning and measuring learning rather than acquisition or proficiency. However, to master something, it needs to be broken down into parts, parts to be mastered. Mastery is gauged by tests.
For this reason I reject the method used in so many school systems called Mastery Learning. Language is not acquired in bits and pieces but through use. Mastery Learning operates against this.
This term should not be confused with the common phrase “master a language” where a high degree of proficiency or fluency is referred to.
Proficiency is an objective in the fl classroom. The goal is to use the language. This is the underlying reason that many of us say that the goal of reading Latin has not been met when learners decode Latin text line by line. That is not reading. Breaking Latin grammar and other features of the language into units that can be mastered militates against the learner acquiring the ability to use Latin. Reading proficiency is what is usually stressed in Latin, although writing can be a course component as well. For those teachers comfortable with spoken Latin, listening and oral comprehension can figure in the course, too, and many teachers feel that oral work helps the learning process, going on the notion that we learn to read languages we speak and understand.
(This was in response to a post on a listserv about proficiency and master learning)