The meaning of the phrase “equal opportunity” is investigated with a combination of historical text studies and philosophical analysis. In the late nineteenth century, this phrase obtained a specialized meaning that cannot be derived from the meanings of its constituent words. The “equality” of equal opportunity refers to equality among people belonging to different groups, and the “opportunities” mostly concern access to desirable activities and resources that are important for a person’s career and chances in life. Contrary to many other social goals, the ideal of equal opportunity does not have a well-defined endpoint, that is, we cannot describe a plausible social state in which equal opportunity is perfectly satisfied. But in spite of this, equal opportunity has sufficient directional precision to tell us in which direction to go. Strivings for equal opportunity form an essential component of strivings for social justice. A major reason for this is that equal opportunities are incompatible with discrimination, i.e., worse treatment of people because they belong to certain groups. Discrimination is a particularly dangerous form of inequality, since each act of discrimination contributes to perpetuating a pattern that can persist for generations. Working for equal opportunity is an efficient way to counteract discrimination.