Measurable indicator for usability

I am starting a new project and I have the problem with the measurability of the requirement usability.

With is an good atomic measurable indicator for usability?

Number of klicks or keystrokes per action, weighted with the estimated relative frequency of the action.

However, this metric does not take into account a few important aspects of usability:

• How easy it is to memorize the sequence of clicks and keystrokes
• The amount of feedback provided to the user
• The likelyhood of invalid inputs

Still it can be useful for comparing user interfaces that are similar with respect to these aspects.