Phenotype refers to any observable characteristic in an organism. The term 'phenotype' may be applied at several levels, ranging from the biochemical (e.g. the 3D structure of a protein), to the anatomical or physiological (e.g. skin colour, height, kidney function) through to the behavioural (e.g. flapping flight or a particular mating ritual) and the products of behaviour (e.g. the construction of a nest or a beaver's dam).

Phenotype is influenced by a multitude of factors; the primary three being genotype, epigenetics and environmental factors. For instance, a phenotype such as body weight could be influenced by the genes concerned with appetite and metabolism, the hormonal (epigenetic) effects on these genes, and by environmental factors such as the availability of food and the frequency of exercise.