An adaptation is a characteristic of an organism that has been favored by natural selection and increases the fitness of its possessor. The concept is central to biology, particularly in evolutionary biology. The Oxford Dictionary of Science defines adaptation as "Any change in the structure or functioning of an organism that makes it better suited to its environment."

Adaptation is the change in living organisms that allow them to live successfully in an environment. Adaptations enable living organisms to cope with environmental stresses and pressures. Adaptations can be structural, behavioral or physiological. Structural adaptations are special body parts of an organism that help it to survive in its natural habitat (e.g., skin colour, shape, body covering). Behavioural adaptations are special ways a particular organism behaves to survive in its natural habitat (e.g., phototropism). Physiological adaptations are systems present in an organism that allow it to perform certain biochemical reactions (e.g., making venom, secreting slime, homeostasis).

Adaptations are traits that have been selected by natural selection. The underlying genetic basis for the adaptive trait did not arise as a consequence of the environment; the genetic variant pre-existed and was subsequently selected because it provided the bearer of that variant some advantage. The first experimental evidence of the pre-existing nature of genetic variants was provided by Luria and Delbrück who developed fluctuation analysis, a method to show the random fluctuation of pre-existing genetic changes that conferred resistance to antibiotics by the bacterium Escherichia coli

While many traits have obvious adaptive purposes, it is worthwhile to point out that many traits are not adaptive, that is, there is no obvious reason scientists can divine for the presence of a certain trait. This situation is common and there are many causes: the utility of a trait is lost and hence does not now appear adaptive, the utility of a trait is unknown, the trait is a consequence of another trait that is adaptive (the Spandrel idea). This observation underscores two other important points: genetic variants arise randomly and hence traits can appear randomly and that because the environment for all living things is constantly in flux, the utility of adaptations will naturally ebb and flow.

Organisms that are not suitably adapted to their environment will either have to move out of the habitat or die out. The term die out in the context of adaptation simply means that the death rate over the entire species (population, gene pool ...) exceeds the birth rate for a long enough period for the species to disappear; due to individual phenotypic plasticity, individuals will be more or less successful. The opposite is selection which in this context means that the birth rate of those carrying the adaptive trait and the hence the underlying genetic variant exceeds over time the birth rate of those that do not carry the adaptive trait .