HMO stands for House in Multiple Occupation and is used to describe houses with multiple households or if you share toilet, bathroom or kitchen facilities with other tenants.
A household is either a single person or members of the same family who live together. A family includes people who are:
- married or living together – including people in same-sex relationships
- relatives or half-relatives, for example grandparents, aunts, uncles, siblings
- step-parents and step-children
This article by Buyassociation explains more. Might be worth looking into.