If "tangential velocity" is not tangental to the orbit, what else can it be tangential to? I studied orbital mechanics in Howard Curtis' Orbital Mechanics for Engineering Students, and while I don't claim that it holds absolute truth, it uses the same definition as the diagram you posted (but less clutter):
(Figure 2.8 from H. Curtis, Orbital Mechanics for ...
Ok so to answer my own question (turns out I just needed to do a little more reading up):
The reference plane for the Azimuth is based on True North with its 0 degree starting at the Meridian line and increasing clockwise 360 degrees until once again reaching the meridian.
It is also possible to use True South to orientate, but then the Azimuth increases ...
During his tenures as Chief of the Astronaut Office and Director of Flight Crew Operations, Deke Slayton had the greatest influence over the titles of astronauts.
NASA's predecessor agency -- the National Advisory Committee for Aeronautics (NACA) -- was more involved in aircraft than spacecraft. Their research resulted in the Bell X-1 supersonic aircraft ...
The technical term for them is "handrail".
Image source: http://www.hunchdesign.com/uploads/2/2/0/9/22093000/restraint_and_mobility_aids.pdf
IVA (IntraVehicular Activity) Handrail if you want to be formal.
Table source: https://snebulos.mit.edu/projects/reference/International-Space-Station/SSP50008RC.pdf