The objective of this review was to clarify what health literacy represents. A systematic review with qualitative syntheses was performed (CRD42017065149). Studies concerning health literacy in all settings were included. Studies before 15 March 2017 were identified from PubMed, Medline, Embase, Web of Science, Scopus, PsycARTICLES and the Cochrane Library. The included literature either had defined the concept of health literacy or made a detailed explanation of health literacy. A total of 34 original studies met the inclusion criteria, including 13 involved in previous systematic reviews and 21 new studies. Health literacy was commonly conceptualised as a set of knowledge, a set of skills or a hierarchy of functions (functional-interactive-critical). The construct of health literacy covers three broad elements: (1) knowledge of health, healthcare and health systems; (2) processing and using information in various formats in relation to health and healthcare; and (3) ability to maintain health through self-management and working in partnerships with health providers. Health literacy is defined as the ability of an individual to obtain and translate knowledge and information in order to maintain and improve health in a way that is appropriate to the individual and system contexts. This definition highlights the diversity of needs from different individuals and the importance of interactions between individual consumers, healthcare providers and healthcare systems.