The appropriate smartass response to "What's up?"
"A localized convention for orienting the somatic bilateral axis. Where the semicircular canals can detect a local gravitational field, this axis tends to align with a vector orthonormal to some gravitational equipotential surface, with the same sense as the direction of field strength attenuation."
You might merely say "The direction of gravitational field strength attenuation" but this will produce a suboptimal annoyance level in the querent.
Allowing for an alternate definition of "is", you may provide some variety by responding with the name of some celestial body such as Mars; however, you will need to refer to a table of ephemerides to ensure accuracy at the moment of response.