If we are referring to what the Italians call "appoggio" which derives from the verb "appoggiare" meaning "to lean", then we must further clarify the term by calling it "abdominal-diaphragmatic support". This term "abdominal-diaphragmatic support" describes the process before and during the production of sound where the singer's diaphragm contracts into a plate formation pushing down/displacing the digestive organs during the inhalation of air into the lungs in order to intake sufficient air to sing a phrase. Right before the production of sound takes place the abdominals contract inwards and upwards, creating a contrary force to the downward/flattening motion of the diaphragm. At this point the diaphragm acts as a stabilizing force on the inward/upward force of the abdominals. This in coordination with full glottal closure/adduction which permits a lean stream/thread of the air supply to be emitted through the vocal folds allows the lungs to be suspended during the production of sound for a longer duration of time.
I'm not sure exactly how one describes support in the exact way that this fill in the blank requires but my guess would be: "Support" can be described as a(n) equilibrium/balanced force between the diaphragm and the abdominals.
I hope this helps!