How many letters does the top answer for “an arc of a circle which is equal to the radius, or the angle measured by such an arc.” have?

The most common answer is radian (6 letters).

Crossword clue: an arc of a circle which is equal to the radius, or the angle measured by such an arc.

1 answer found in Letterora's clue database (Wikidata facts, Open Trivia DB, WordNet and Webster definitions — all licensed sources).

radian 6 letters web-def

the unit of plane angle adopted under the Systeme International d'Unites; equal to the angle at the center of a circle s