It is geometry that is not controlled by a feature. For example, start the line command and turn off the "associative" option; create a line and press OK. You will have a line object that you can use, but no line feature in the part navigator.
Each feature has a "timestamp" (the number after the feature name); when NX updates the model, the features are applied in timestamp order.
It sounds similar to what you get if you use "Remove Parameters".
In fact I just tried it and indeed "Remove Parameters" when I did that to a line creates a line in Non-timestamp Geometry. It gets removed from "Model History" of the model navigator, and goes to Non-timestamp Geometry. Makes sense because now there is no "history" on how that entity was created.
What's interesting is doing "Remove Parameters" on a solid, say an Extrude created in the Modeling tab creates a "Body(nn)" in the Model History and not in Non-timestamp Geometry.
Is there are reason for that????
"Remove parameters" is not a time-stamp operation. It just lobotomizes the body and leaves the body as a simple, unparameterized "body" feature. there is no timestamp, and no way to delete or suppress a deparameterization.
Why would anyone do such a thing as remove parameters? I could write a whole thesis on that.
Honesty may be the best policy, but insanity is a better defense.