Sexism

In theory

Sexism refers to gender-based discrimination or prejudice, typically favoring one sex/gender and disregarding or demeaning the other sex/gender. The concept of sexism belongs to the group of -isms that refer to systemic inequalities between certain groups of people based on (perceived) characteristics. These inequalities operate on different levels: the individual, institutional and cultural level. They consist of beliefs, practices, and social structures. As is stressed by Barreto and Ellemers (2014):

1. -Isms do not depend on the intentions of the perpetrator. For instance, a person can hold sexist attitudes, or behave in a sexist manner, without having the intention to do so.
2. To be classified as sexist, racist, etc, incidents do not need to produce direct harm. Some incidents (e.g. micro-aggressions) do not directly harm an individual, but they do contribute to the systematic oppression of a certain group of people.

In practice 

Sexist, racist, ageist, ableist, etc. beliefs, practices and structures may be hard to recognize, as they are deeply embedded in our culture and everyday lives. Moreover, as these -isms operate on different levels, and consist of different types of practices, they are hard to eliminate.