A simple definition of inequality from Merriam-Webster's Dictionary:
an unfair situation in which some people have more rights or better opportunities than other people
From the World Book Online:
Sex discrimination is the unequal treatment of individuals based on their sex. Laws and traditions have helped create and maintain political, economic, and social inequality between the sexes in most societies throughout history. Sex discrimination has limited the opportunities of both women and men, but especially those of women. Until the early 1900's, for example, nearly all countries denied women the same voting rights as men. In many countries, women still hold few or none of the most powerful jobs and political offices. This inequality exists even though women make up about half the population of nearly every country.
Mezey, Susan Gluck. "Sex discrimination." World Book Student. World Book, 2016. Web. 8 Feb. 2016.
Educational Inequity from Humanium.com
For many children who still do not have access to education, it is notably because of persisting inequality and marginalization. In developing and developed countries alike, children do not have access to basic education because of inequalities that originate in sex, health and cultural identity (ethnic origin, language, religion). These children find themselves on the margins of the education system and do not benefit from learning that is vital to their intellectual and social development.