It's a buffer assembly, and the specific part of the assembly that is a spring, is identified as an action spring. It has been so since it was invented, and is identified as such in every Marine Corps manual I have ever seen. I am pretty sure the Army uses the same terminology, as quite a number of parts manufacturers list action spring as part of the description.
But, usage of the term buffer spring is common, though my guess it originated as some sort of slang term from buffer assembly spring. But reality, the spring does not "spring the buffer", it springs the action, hence it is an action spring.
