Plain English Meaning
It is the official rulebook that says aircraft breathing oxygen has to be very pure and very dry. Pilots use this standard to make sure the oxygen put into their airplane is the right kind for flying.
Full Definition
An SAE (Society of Automotive Engineers) Aerospace Standard that defines the purity and water-content requirements for Aviator's Breathing Oxygen (ABO) used in aircraft oxygen systems. It specifies that the oxygen must be at least 99.5% pure by volume and contain no more than a small, defined amount of water vapor, so that it remains safe to breathe at altitude and will not freeze and block oxygen lines or regulators in cold conditions.
Context Anchor
Seen in oxygen system discussions and when checking what type of oxygen may be used to fill an aircraft oxygen bottle.