Mental Availability is not awareness

A brand's mental availability refers to the probability that a buyer will notice, recognize and/or think of a brand in buying situations.  It depends on the quality and quantity of memory structures related to the brand.  See chapter 12 of "How Brands Grow".
A brand's mental availability refers to the probability that a buyer will notice, recognize and/or think of a brand in buying situations.  It depends on the quality and quantity of memory structures related to the brand.  See chapter 12 of "How Brands Grow".
So this is much more than awareness, whether that is top-of-mind awareness, recognition or recall.  Indeed all of these measures are flawed by the use of a single, a-situational, cue (usually the product category name, i.e. what the marketer calls the product category).
And mental availability is not attitude.  It's not about what consumers like about the brand, or not.  Though the better a consumer knows a brand the better they tend to feel about it - familiarity breeds contentment.
A brand's availability varies across situations, so higher mental availability means being easily noticed and/or thought of in many different buying situations.  Some brands do well in some particular situations, some do well in many situations.  Some do well with a few consumers, some do well with many consumers.  The easier the brand is to access in memory, in more buying situations, for more consumers, then the higher the overall mental availability.
Together this all means that advertising to refresh and build mental availability requires more than merely reminding consumers that the brand exists, but that's another story.