The Asexual Visibility and Education Network defines an asexual as “someone who does not experience sexual attraction” and stated, “[a]nother small minority will think of themselves as asexual for a brief period of time while exploring and questioning their own sexuality” and that “[t]here is no litmus test to determine if someone is asexual. Asexuality is like any other identity — at its core, it’s just a word that people use to help figure themselves out. If at any point someone finds the word asexual useful to describe themselves, we encourage them to use it for as long as it makes sense to do so.” (From Wikipedia)
According to various surveys, some of which had quite large data sets, one person in a hundred never feels sexual attraction to anyone. Asexuals can have sex out of curiosity or to please a partner, so lack of sexual activity is not in itself the criterion. More info is here.