Obsessional describes a feeling or habit that is like an obsession: an extreme, unhealthy interest or worry that takes over the mind so a person cannot think about much else. Doctors and psychologists use this word a lot — obsessional thoughts, obsessional habits. In ordinary conversation people more often say obsessive, and the two sit very close. Obsessional can sound a little more like a clinic or a textbook. It is not praise. It points to something that has gone too far and is crowding out the rest of life. The noun for the thing itself is obsession; this word describes that kind of thinking or behaviour.