The user-interaction tag is one of our most common tags but it's not a term I've ever heard before this site. I'm thinking maybe people use it as a synonym of interaction-design but user interaction sounds more like a way of saying "human computer interaction," which on this site is a uselessly broad tag.
Should we nix or synonmize this tag? I'm learning towards nixing it as I bet a lot of people just type "user" into the tag bar and see user-interaction pop up and think "gee, that applies to me!"
If it means what I think it means, eg. the way users interact with software, then it's too broad. It looks like it's on quite a few questions that are also tagged with things like button and forms. Those are specific and in line with the nature of the questions. So I think it might be okay to get rid of it and rely on those more specific tags instead.
It is probably fair that "user-interaction" is too broad. I think "interaction-design" focuses it far more on the design of the interactions.
User-behaviour - as mentioned by @Patrick - is probably sufficiently different, as it should be about how users behave - more anthropological or psychological. I am all for leaving some of these similar words, as it can help questioners to think about where their question sits. The user-interaction should fit with 90% of the questions, and so does not seem helpful in that.