In this context, genus is the minimal genus taken over all Seifert surfaces of the knot (i.e. over all oriented spanning surfaces of the knot). Ozsvath and Szabo prove (in this paper) that the genus of a knot is the maximum Alexander grading in which the knot Floer homology is non-trivial. If the knot Floer homology of a knot is thin, then the degree of the Alexander polynomial is equal to the maximum Alexander grading in which the knot Floer homology is non-trivial (and thus it is also equal to the genus of the knot).