Definition 1 of 2
Plain English Meaning
Extra drag created where two parts of the aircraft meet and the air flowing around each one collides and gets messy.
Full Definition
Drag generated by the mixing of airflow at the junctions where two parts of an aircraft meet, such as where the wing joins the fuselage or where a strut meets a wing. The airflow patterns from the two surfaces interfere with each other, producing additional drag beyond what either surface would create on its own.
Context Anchor
Seen in airframe design, inspection, fairing installation, and discussions of aircraft performance.
Derivation
Interference' comes from the Latin 'inter' (between) and 'ferire' (to strike) — literally 'to strike between.' It captures the idea of two airflows running into each other at a junction and disturbing each other.