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Abstract Behaviours designated as basking span a broad array of ectothermic and endothermic animals. Understanding the role that basking plays in thermal biology is important because maintaining body temperatures within certain limits is essential to survival, growth and reproduction, and may affect responses to climate change. In the case of turtles and tortoises, behaviours designated as basking may occur out of water, at the surface of a water body or deep in a water body, during the day or at night, and with or without exposure to solar radiation or another heat source. On the available evidence, the primary function of most nominal chelonian basking is thermoregulatory warming, which provides numerous benefits including expediting gonadal recrudescence, enhancing foraging, feeding, digestion and growth, and amelioration of infectious disease and endoparasitism. Substantial evidence supports conditioning of the skin and shell as a common additional function of chelonian basking. Other functions of nominal chelonian basking with some supporting evidence for particular species and environments are thermoregulatory cooling, leech removal, vitamin D synthesis and osmotic control. Nominal chelonian basking may sometimes be undertaken for resting or avoiding aggression, but supporting evidence is scant. The only uniting feature of all chelonian behaviours designated as basking is quiescence, but not all quiescent chelonian behaviours are labelled as basking. Consequently, the general meaning of chelonian basking is indeterminate. Avoiding ambiguity requires the provision of explicit criteria by which basking can be distinguished from other quiescent behaviours, and the attachment of qualifiers that specify the applicable time of day, medium and environmental exposure.