A practice in which the most passionate of Harry Potter fans engage, in which upon first meeting another person one attempts to ascertain which house in Hogwarts that person belongs before learning anything else about said individual. Those who advocate for this practice claim it is far more effective in predicting and anticipating a person's character and personality than other forms of "marking" that many other humans may deploy. For example, the feminist philosopher Marilyn Frye describes "sex marking," a process of trying to determine another's sex and/or gender so as to impose upon them rigid expectations based on inaccurate gender norms and roles that only serve to stifle human capacity. Male and female are ambiguous, broad, and essentially meaningless term, containing within each of them the full spectrum of the human condition. Sex marking, therefore, is an inaccurate means of developing an understanding of another human. House marking, on the other hand, is far more effective in anticipating how a person will act, think, or live. Knowing someone is a Slytherin provides a far better basis for predicting behavior than knowing they are female.
I couldn't help "house marking" that guy I met last night I could tell he was a total Slytherin!