In simple terms, essentialism is a generalization stating that certain properties possessed by a group (e.g. people, things, ideas) are universal, and not dependent on context. For example, the statement 'all human beings are mortal' is essentialist.
According to essentialism, a member of a specific group may possess other characteristics that are neither needed to establish its membership nor preclude its membership, but that essences do not simply reflect ways of grouping objects; they also result in properties of the object, as the object can be subjugated to smaller contexts.
Anthropology professor Lawrence Hirschfeld gives an example of what constitutes the essence of a tiger, regardless of whether it is striped or albino, or has lost a leg. The essential properties of a tiger are those without which it is no longer a tiger. Other properties, such as stripes or number of legs, are considered inessential or 'accidental'.  Biologist Ernst Mayr epitomizes the effect of such an essentialist character of Platonic forms in biology: "Flesh-and-blood rabbits may vary, but their variations are always to be seen as flawed deviation from the ideal essence of rabbit". For Mayr, the healthful antithesis of essentialism in biology is "population thinking".
This view is contrasted with non-essentialism, which states that, for any given kind of entity, there are no specific traits which entities of that kind must possess.
Essentialism came under scrutiny and criticism in the mid to late 20th century by the American pragmatist Richard Rorty