A trait is a well-defined, measurable characteristic (morphological, phenological, physiological, behavioural or cultural) of an individual organism. Traits are usually measured at the individual level and used comparatively across species or other relevant levels of organisation. While the definition implies that traits are quantitative variables (e.g. body mass, carbon content), qualitative or categorical traits (e.g. feeding mode or reproductive strategy) are also often used to describe organisms.