If this is a piece of socio-historical scholarship, alterity is is the correct translation of alteridad. It means otherness, but it has a specific philosophical meaning that is lost in simply translating it as otherness. The idea is that the otherness--or absolute difference of the nature of the feminine from the masculine--is part of what defines masculinity. The writer is no doubt bulding on the philosphical work of thinkers like Emmanuel Levinas, Enrique Dussel, and Jacques Derrida, which has given rise to a whole genre of scholar that builds on this notion (most of it "post-modern". I am absolutely confident about this because I teach university graduate seminars that use readings in Spanish and French, as well as English by these scholars on precisely these themes. The sentence below I'm including as an example comes from Dussel's "The Invention of the Americas: Eclipse of the Other and the Myth of Modernity" (1995). This theme of alterity has also become very pronounced in feminist scholarship in recent years. The two web references illustrate how widespread this concept has become in scholarly discourse in philosophical and feminist work.