Definition
To come together and merge into a single, larger mass. In aviation contexts, the term most often refers to small water droplets in clouds joining to form larger droplets heavy enough to fall as precipitation, or to fuel and oil droplets combining inside filters and separators so they can be drained or removed.
Plain English
Small things of the same kind come together and combine into one bigger thing.
Context Anchor
Seen in weather discussions about cloud formation, rain, drizzle, and how precipitation develops.
Derivation
From the Latin coalescere, meaning 'to grow together' — co- ('together') plus alescere ('to grow'). The image is of separate things growing into one, which is exactly what happens when tiny droplets merge into a single larger one.
Why Pilots Care
Water in fuel can cause engine failure; coalescing filters ensure safe removal of contaminants.
Grounding Statement
Picture two raindrops on a windshield drifting into each other and forming one larger drop — that is coalescing.
Intuition Check
Coalesce does not just mean “move close together.” In this context, the separate droplets actually join and become a larger droplet.
Example Sentence 1
Inside a developing cumulus cloud, tiny droplets coalesce into larger drops that eventually fall as rain.
Example Sentence 2
In the carburetor heat system, moisture can coalesce on cold surfaces leading to icing.