The term "non-traditional students" is commonly used in higher education research and yet its definition has been unclear. This study systematically reviewed 45 definitions of "non-traditional student" in mental health research conducted within the higher education context using a standardised data extraction and appraisal tool. Findings suggested a wide range of variations on how this term was defined. Thirteen different categories of meaning have been used, including age, multiple roles, mode of study, gap in studies, commuter status, being demographically "different" from the norm, sex, admission pathway, enrolment in "non-traditional" programs, being "disadvantaged", disability and trauma, ethnicity, and having a previous degree. Different combinations of categories were mentioned in the reviewed definitions and wide variations existed within each category of meaning. The term "non-traditional student" does not currently represent a functional category in communicating a distinct concept. Future research should improve the clarity and consistency in which it is defined.