Following is a collection of terms which I believe are vague and thereby void of meaning in common usage. That is not to say that they should be avoided entirely necessarily, but may require clarification when used.

Usability

"This piece of software is more usable than this other one." Without the use case (by whom and for what purpose), meaningless. The implied use case may be "for me" or "for average Joe User".

To be more specific, use:

• Learnable
• Accessible
• Efficient to use

Note that these terms may also need some qualification.

Learnability

I define a system as "learnable" if a particular user (demographic) can learn how to use it to a particular level within a particular amount of time, given a particular set of training materials. Thus, it could be defined by the (Python) function islearnable(by_whom, to_what_degree, within_how_long, with_which_training_resources).