Final answer:
The opposite of existentialism is essentialism, which emphasizes fixed and unchanging essences or qualities.
Step-by-step explanation:
The term that applies to the opposite of existentialism, which was assumed by most people until around the 20th century, is essentialism.
Existentialism emphasizes individual existence, freedom, and choice, while essentialism focuses on the belief in fixed and unchanging essences or qualities that define things or individuals.
For example, while existentialism asserts that human freedom is a defining characteristic and that individuals create their own meaning and purpose in life, essentialism would argue that there are inherent qualities or purposes that define human beings.