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Fit is a versatile term with several related meanings in everyday language, science, and industry. As an adjective, it commonly denotes physical health or conditioning: a person who is fit is in good shape. It also means suitable or appropriate for a particular purpose or circumstance, such as a device that is fit for use or clothing that is the right fit for someone. The verb form to fit means to be the right size or shape for something, or to join parts so that they match or function together.

In everyday usage, fitness emphasizes physical activity, nutrition, and lifestyle factors that influence health and performance. Fitness is often described in terms of attributes such as strength, endurance, flexibility, body composition, and cardiovascular health. Fitness training, assessment, and related services make up a major industry and public health focus in many regions.

In scientific contexts, fit describes how closely a model or theory aligns with observed data. A statistical model is said to fit the data well if its predictions correspond to the measurements. The term is paired with phrases like goodness of fit and fit statistics. In biology, fitness refers to the reproductive success of organisms and their contribution to future generations, a central concept in evolutionary theory.

Other uses of fit include social and functional compatibility, as in “fit in” with a group or “fit” of components within a system. The phrase “a fit” can also denote a temporary episode, such as a fit of anger, coughing, or a seizure, depending on context. Clothing and fashion settings use fit to describe how garments conform to the body.