Definition
Any liquid having a flash point below 100°F (37.8°C), meaning it gives off enough vapor at relatively low temperatures to ignite when exposed to an ignition source. Aviation gasoline and Jet A fuel are common examples carried and handled in aircraft operations.
Plain English
A liquid that can catch fire easily because it releases burnable vapors at normal or near-normal temperatures.
Context Anchor
Seen in aircraft fueling, maintenance shops, storage areas, safety data sheets, and fire-safety procedures for fuel, solvents, and cleaners.
Derivation
From the Latin flammare, meaning 'to set on fire.' The term emphasizes that it is the vapor coming off the liquid — not the liquid itself — that actually burns.
Why Pilots Care
Improper handling risks fire or explosion during refueling and storage operations.
Grounding Statement
If a small amount of fuel or solvent is spilled in a warm hangar, the invisible fumes above it may be the part that catches fire first.
Intuition Check
Do not think only the liquid surface is the danger. With a flammable liquid, the vapors above the liquid are usually what ignite.
Example Sentence 1
Avgas is a flammable liquid, so the line crew bonds the fuel truck to the aircraft before refueling to prevent a static spark.
Example Sentence 2
All containers of flammable liquid must be stored away from ignition sources in the hangar.