Gender essentialism starts with the mistaken belief that gender is a fundamental and immutable attribute of human beings, and proceeds to draw from this (combined with largely anecdotal observations about human behavior within the observer's culture) a number of conclusions about the nature of "femininity" and sometimes "masculinity".
These conclusions are in many ways deeply embedded in Western culture, due largely to their utility (from an authoritarian perspective) in controlling and limiting the range of culturally-acceptable actions and in maintaining a patriarchal system in which men have power over women.
Among these are the belief that there are exactly two genders (male and female), and that the condition of being male or female carries with it a certain number of immutable characteristics. Gender fundamentalism (a view held by many social conservatives) additionally holds that individuals have a social obligation to carry out cultural roles which have been assigned to their gender.
Gender essentialism is sometimes erroneously referred to as "biological essentialism", a related but distinct concept. Within the context of feminism it may be shortened to just "essentialism", although there are other types of essentialism.