In philosophy, essentialism is the view that, for any specific kind of entity, there is a set of characteristics or properties all of which any entity of that kind must possess. Therefore all things can be precisely defined or described. In this view, it follows that terms or words should have a single definition and meaning.
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 essentialist statement 'all human beings are mortal'.
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.
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'. 
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 late 20th century, particularly by social scientists. Discussion of its possible limitations has taken place among biologists as well.
An essence characterizes a substance or a form, in the sense of the Forms or Ideas in Platonic idealism. It is permanent, unalterable, and eternal; and present in every possible world. Classical humanism has an essentialist conception of the human being, which means that it believes in an eternal and unchangeable human nature. This viewpoint has been criticized by Marx, Nietzsche, Sartre, and especially Stirner, as well as many other modern and existential thinkers.
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