"Usability equals predictability."
As in, if you can accurately predict what's going to happen next in an
interaction, it's because the action you're taking is understandable, clear,
logical, makes you feel confident, etc. If you can accurately predict what's
next, the interaction has high usability. If you can't accurately predict
what's next, the interaction has low usability.
Shoot holes in that statement.
-r-