Definition
A propeller whose blades have been rotated to an angle approximately parallel to the oncoming airflow, presenting the thinnest possible edge to the relative wind. This position stops the propeller from rotating (or nearly so) and minimizes drag when an engine has failed or been shut down in flight. Feathering is normally available on multi-engine aircraft and on some single-engine turboprops, and is accomplished through the propeller governor or a dedicated feathering control.
Plain English
A propeller whose blades have been turned edge-on to the wind so it stops spinning and creates as little drag as possible. Pilots feather the prop on a failed engine so the dead engine doesn't drag the airplane through the sky.
Context Anchor
Seen in multiengine aircraft procedures, engine-failure checklists, and discussions of propeller control after an engine shutdown.
Derivation
From the verb 'to feather,' originally a rowing term meaning to turn an oar blade flat as it leaves the water so it slices through the air rather than catching it. The aviation use carries the same idea: turn the blade edge-on so it slips through the air instead of resisting it.
Why Pilots Care
Feathering greatly reduces asymmetric drag, allowing the aircraft to maintain better climb performance and control on the remaining engine.
Analogy
Think of holding your hand out a car window. Palm flat to the wind, it gets shoved back hard. Turn your hand sideways, and the wind slips past with almost no push. Feathering does the same thing to the propeller blades.
Intuition Check
Feathered does not mean decorated with feathers or lightly touched. In this aviation use, it means the propeller blades are turned edge-on to the airflow to reduce air resistance.
Example Sentence 1
After confirming the right engine had failed, the pilot feathered the propeller and began the single-engine climb checklist.
Example Sentence 2
During the emergency procedure, the crew confirmed the propeller was fully feathered before shutting down the engine.