Plain English Meaning
A mask that fits over your mouth and nose together, so all the air you breathe in comes from the oxygen system.
Full Definition
A style of oxygen mask that covers both the mouth and the nose, sealing against the face so that the wearer breathes the supplied oxygen rather than ambient cabin air. Oronasal masks are commonly used with continuous-flow and diluter-demand oxygen systems at the altitudes where supplemental oxygen is required.
Context Anchor
Seen in aircraft oxygen system discussions when describing the kind of mask used to deliver breathing oxygen to a pilot or passenger.
Derivation
From Latin 'os, oris' meaning mouth, and 'nasus' meaning nose. 'Oronasal' literally means 'mouth-and-nose,' which describes exactly what the mask covers — useful because other oxygen delivery types (like nasal cannulas) cover only the nose.