How does in memory testing differ from using Mocks

So i've seen the term "in memory" testing used quite a bit. I always took it to mean that things like the API's and Databases were running locally on the machine that's running the website for example on IIS. And that you test against this website.

So say I mocked the API's and had the site running locally on IIS and proxy to the mock, that responds with generated or canned responses. If I tested against this site, would this be considered "in memory" testing.

I cant see how that can be referred to as in memory testing when only the website is running in memory.

Have I got the definition wrong?