I think a clear definition would be helpful, because there does seem to be some confusion around the term. Here's what I've found to be of note:

1. The term "treatment-resistant" specifically describes major depressive disorder that has not been relieved by full cycle use of at least two different anti-depressants. That's actually the entire definition. (Source.)

2. A person is not treatment-resistant. This is important to note, as saying "I'm treatment resistant" or "you're treatment resistant" implies a personal fault that the definition is clearly not meant to imply.

3. What seems to often be meant by some in their use of the term treatment-resistant, is the phenomena of psychological resistance in therapy, however when that becomes maladaptive over time it is actually considered to be the result of behaviors exhibited by both the subject and the therapist. (Source.) I mention this because it's interesting the degree of sole personal fault people often associate with this phenomena as well.

I had never thought about it that much to look into the definition further until I came to this forum, and heard people saying it about themselves and one another in a way that didn't make sense to me, as however consumed we may be by our condition at times: we are not our condition.