This post covers the scientific and legal definitions of sexism, sexual harassment and sexual discrimination. We include an overview of the different ways in which sexism is described, such as hostile, benevolent, accidental or unintentional. These qualifiers of sexism can sometimes confuse people, as they invite people to see sexism as an individual or subjective idea. Sexism is neither – it is about how the collective interactions that happen at the everyday level are connected to institutional practices of harassment and discrimination. We provide examples of how sexist culture operates in at various levels of STEM, from undergraduate courses to gender inequality in pay, science publishing and recognition of women’s achievements. STEM Women seeks to move beyond superficial arguments about what sexism is and isn’t. The scientific evidence, some of which is included here, has established that inequality exists. We are looking for practical solutions to address inequality and lift the participation of women in STEM.