Definition of FONTANEL. : a membrane-covered opening in bone or between bones; specifically: any of the spaces closed by membranous structures between the uncompleted angles of the parietal bones and the neighboring bones of a fetal or young skull.See fontanel defined for kids. a membrane-covered opening in bone or between bones; specifically: any of the spaces closed by membranous structures between the uncompleted angles of the parietal bones and the neighboring bones of a fetal or young skull.