I recently passed the bar exam of the state in which I want to work, and sent an inquiry email to a partner of a mid-law firm. she responded immediately saying that she is not sure if they will be hiring within the next six months. she added that she is willing to meet with me, but with no assurance that they will be able to offer me a place.

Is this what is known as as 'informational interview"

Yes.

Even though she cannot commit to anything, many opportunities come from info interviews. It may not be today; it may not be with that firm; but it's better than being a nameless/faceless resume when they do decide to hire.

from what OP said, it sounds more certain than an informational interview but less certain than a regular interview.

If it's not a hiring interview, it's an info interview. OP said nothing is certain other than getting to talk with the partner--but that's OK because other benefits flow from info interviews.

No. The primary characteristic of an informational interview is that the potential job seeker is the one actively gathering information. The stated purpose is for that person to learn more about the firm/partner/whatever. This sounds like a preliminary evaluation interview. There is no defined job on the line, but it sounds like the OP should be prepared to be actively interviewed, not be the one that is doing the interviewing.

OP here. Thanks. I will actively prepare for it then.
this is my dream firm.
I thought i was being sensitive.
The partner emphasized that if she did not know whether the firm will be in a position to hire someone and so cannot offer me assurances.
actually it doesn't make a difference for me - because even if they are actively hiring I may not get the job anyway.

She also noted talked a little about the practice areas of that particular office, which she thought was slightly different from what I had expected.

I disagree with your characterization, because info interviews have much deeper purposes despite the plain language. Whatever you want to call it--informational or otherwise--OP needs to prepare for any interview as if a job offer is on the line.