Essentialism, at its simplest, is the view that things have essences (the attribute, or set of attributes, that make an object or substance what it fundamentally is). Thus, for any specific kind of entity, there is a set of characteristics (or properties or traits), all of which any entity of that kind must have. A member of a specific kind of entity may possess other characteristics but these neither establish nor preclude its membership.

It is contrasted with Non-Essentialism (which states that there are no specific traits which any given kind of entity must have), and with Nominalism (which states that abstract concepts, general terms or universals have no independent existence but exist only as names).

An essence characterizes a permanent, unalterable and eternal substance, or a form (in the sense of the Forms or Ideas in Platonic Realism). Plato was therefore one of the first essentialists, believing in the concept of ideal forms, an abstract entity of which individual objects are mere facsimilies. Classical Humanism has an essentialist conception of the human being, which means that it believes in an eternal and unchangeable human nature.

• Mereological Essentialism is the view that objects have their parts essentially. Therefore, if an object loses or gains a part, it would effectively cease to exist in that it would not be the same object anymore.
• Ethical Essentialism (or Moral Absolutism) is the claim that some things are wrong in an essential or absolute sense, breaking a universal, objective and natural moral law and not merely an adventitious, socially or ethically constructed one.
• Epistemological Essentialism is the view that all entities have intrinsic properties that can be discerned by reason (sometimes attributed to Aristotle).
• Sociological Essentialism is a sociological (as opposed to philosophical) theory which states that positions on gender, sexuality, race, ethnicity or other group characteristics are fixed traits, not allowing for variations among individuals or over time. It has been used, at different times, as a convenient doctrine by both nationalist and liberationist movements, and for simplifying the task of colonization and imperialism.
• Educational Essentialism is an educational (as opposed to philosophical) theory that states that children should learn the traditional basic subjects and that these should be learned thoroughly and rigorously. An essentialist program normally teaches children progressively, from less complex skills to more complex.