parsec

Concise Oxford English Dictionary © 2008 Oxford University Press:
parsec /ˈpɑːsɛk/

▶noun Astronomy a unit of distance equal to about 3.25 light years (3.08 × 10¹⁶ metres), corresponding to the distance at which the mean radius of the earth's orbit subtends an angle of one second of arc.

– origin early 20th cent.: blend of parallax and second2.