Department of spell-checking

The word 'alterity' isn't recognized by Microsoft Word's spell check.  A quick gander at the OED entry tells me that the word became more widely used in 20th century critical and cultural theory (not something that the writers of whatever dictionary Microsoft uses are too familiar with, I guess), but at any rate, the earliest quotation is from 1425:

Speculum Sacerdotale 234   Sche [sc. woman] made an alterite and an oþerhede in that tyme, that sche made alienacion and partynge bitwene God and man.

 I also found an awesome (now obsolete) synonym in differency.  I think I'm going to incorporate that into my vocabulary after this.  Or not.