Definition
The gaseous (vapor) state of water, formed when liquid water absorbs enough heat to change phase. In aviation systems, steam carries large amounts of energy and can occur in powerplant cooling jackets, deicing systems, and as a byproduct of combustion in the exhaust.
Plain English
Steam is water that has turned into gas because it got hot enough to boil. It holds a lot of heat energy in that gas form.
Context Anchor
Seen in aircraft maintenance, cleaning, heating, and discussions of moisture, condensation, or hot water vapor around equipment.
Derivation
From Old English 'steam' meaning vapor or fume rising from a hot substance. The aviation use stays close to this original meaning -- water vapor produced by heat.
Why Pilots Care
Steam can burn skin, fog windows, or indicate heat and moisture where they should be controlled. Around aircraft, it should not be confused with smoke, which may point to fire or overheated material.
Analogy
A boiling kettle gives a useful picture: the hot water becomes vapor, and the white cloud near the spout appears as that vapor cools in the surrounding air.
Grounding Statement
Think of the white plume rising from a kettle just after it boils -- that visible cloud is condensing steam.
Intuition Check
Do not treat steam as the same thing as smoke. Steam is water-based; smoke comes from something burning or overheating.
Example Sentence 1
Water injected into the cylinders during high-power operation absorbs heat and turns to steam, helping cool the charge.
Example Sentence 2
The mechanic watched steam rise from the exhaust during the ground run-up check.