technically, a hybrid is part gas powered, part electrical powered vehicle
in our terms, hybrid usually implies a car that no longer has the original spec motor in it, but has been replaced with something bigger, or better, or both.
ie:
-sbc -small block chevy
-sbf -small block ford
-sbm -small block mopar
-bbc -big block chevy -rat motor
-bbf -big block ford
-bbm -big block mopar - elephant (if it's a hemi, but bbm usually doesn't fit into a Z with stock strut towers)
-turbo nissan motors from japan.
-nissan motors generally not present for your model at time of its production (ie. 280zxt in a 240, vg30dett in a z31, etc)
-wankel motors
-etc
to answer your question, your car is not a hybrid. That motor was originally available as an option for your car, and is a direct bolt in. This would be applicable to swapping a vg30dett into an n/a z32, 280zxt motor into an n/a 280zx, and so forth. vg30dett would be a hybrid in your z31