The term ‘STEM literacy’, while often used as a slogan for the goal of STEM education, is open to multiple interpretations and used without much research evidence to support its validity. The purpose of this paper is to review the research literature in science, technology/engineering and mathematics education to examine how literacy in each respective discipline has been defined, conceptualised and studied. The literacies across the disciplines are then compared to identify similarities and differences in order to determine on what basis these literacies can be conceptualised collectively, or not. Based on the similarities found in several language and thought processes of the disciplines, we conclude that there is presently a research basis for postulating a unitary STEM literacy that reflects the shared general capabilities required in all the STEM disciplines. At the same time, there are also substantial differences that support the retention of the existing literacy constructs (i.e., S.T.E.M. literacies) to reflect the specific linguistic, cognitive and epistemic requirements found in each disciplinary area. This distinction from the singular STEM literacy is necessary to highlight the skills and practices that are unique to each particular discipline, and therefore not applicable in all the other disciplines. Given the haphazard rhetoric regarding STEM education, what STEM literacy comprises requires analysis and clear articulation that is framed by literacy research and scholarship in each STEM discipline.