Definition
An instrument in a supplemental oxygen system that indicates the rate at which oxygen is being delivered from the storage cylinder to the user, typically calibrated in liters per minute or matched to altitude markings so the pilot can verify proper oxygen flow.
Plain English
A small gauge that shows how fast oxygen is flowing through the line to your mask, so you can check that you are actually getting the oxygen you need.
Context Anchor
Seen during oxygen-system preflight checks and while using supplemental oxygen at altitude.
Derivation
Flow comes from an Old English word meaning to move like a stream. Meter comes from a Greek word meaning measure. Together, flow meter means a device that measures moving fluid or gas, which fits its job of showing oxygen movement in the system.
Why Pilots Care
Correct flow ensures adequate oxygen reaches the body and prevents hypoxia at altitude.
Intuition Check
Do not read meter as a distance meter or just as an on/off light. Here, it means a device that shows oxygen flow; it does not by itself prove the person is breathing the oxygen correctly.
Example Sentence 1
Climbing through 12,500 feet, the pilot turned on the oxygen system and checked the flow meter to confirm a steady delivery rate before donning the mask.
Example Sentence 2
During the oxygen system check, the flow meter needle rose steadily when the valve was opened.