We notice that the "password security" discourse is missing the most fundamental notion of the "password strength" -- it was never properly defined. We propose a canonical definition of the "password strength", based on the assessment of the efficiency of a set of possible guessing attack. Unlike naive password strength assessments our metric takes into account the attacker's strategy, and we demonstrate the necessity of that feature. This paper does NOT advise you to include "at least three capital letters", seven underscores, and a number thirteen in your password.