Amount of reverse transcriptase in µg or mM for qRT-PCR
Reverse transcriptase (and most other commercial enzymes these days) is made recombinantly, then purified to varying degrees, using varying methods, among different suppliers (and sometimes between different lots from the same supplier). Some enzymes actually have multiple functional units combined in a complex, others may require a post-translation activation step like phosphorylation by a separate kinase in order to be fully functional. All of these variables can yield preparations of different molar concentration, but very similar activity. Therefore, companies like New England Biolabs have unit definitions for their various products based on activity.

Having dealt with NEB in the past, I know that they generally have a concentration for their products as well, so if you're doing experiments like an enzyme inhibition assay, you can dose for concentration as well as activity. It will vary from enzyme to enzyme whether your inhibitor will even bind to any inactive protein that may be present, in which case activity would be the better attribute to use in your calculations.