1939, in educational jargon (opposed to progressivism), from essential + -ism. Related: Essentialist.

Defining a group of people by one -- or a small set of -- fixed properties. Gender, religion, race or sexual orientation are the most common properties. It assumes that there is no possibility of vari [..]

The philosophical view that all members of a class of objects (such as a species) share certain invariant, unchanging properties that distinguish them from other classes. © 2005 by Sinauer As [..]

Philosophy. Where things have essences that makes them what they are.

taken from Women Studies page of Drew University - "The belief in a uniquely feminine essence, existing above and beyond cultural conditioning...the mirror image of biologism which for centur [..]

the idea that all things have an underlying or true essence. Racial essentialists argue that all members of a specific racial group share certain basic characteristics or qualities that mark them as inherently different from members of other racial groups.

The notion that there are basic, unvarying, elements that determine, or strongly affect, the behaviour of people and social systems (e.g. the idea that there are inherent differences in the behaviour [..]

Essentialism is the view that entities have some immutable core or characteristic that defines the entity but tht is not always apparent.

is the generally false belief that there are certain “essential,” innate human traits that are unchanged by history, as opposed to a "social constructionist" view that traits that seem essential were in fact created over time through social interactions that can therefore be changed. Racial essentialists argue that all members of a racial [..]

The belief that people and/or phenomenon have properties that are essential to what they are. In a feminist context, the belief in a unique and unchanging feminine essence existing above and beyond cu [..]

the essence or "whatness" of something.  In the context of race, ethnicity, or culture, essentialism suggests the practice of various groups deciding what is and isn't a particu [..]

the idea that people, objects, and ideas can be identified based on externally observable features. Although this is empirically true, social justice warriors consider this idea to be problematic.

The philosophical view that all members of a class of objects (such as a species) share certain invariant, unchanging properties that distinguish them from other classes.

The belief that people and/or phenomenon have properties that are essential to what they are. In a feminist context, the belief in a unique and unchanging feminine essence existing above and beyond cu [..]