means A unit of astronomical distance equal to about 3.26 light-years, defined as the distance at which one astronomical unit subtends an angle of one arcsecond.

from A compression of 'parallax of one arcsecond,' coined in the early 20th century by the astronomer Herbert Hall Turner. The logic is geometric: as Earth circles the Sun, a nearby star appears to shift slightly against the distant background — that tiny wobble is parallax. A star whose parallax measures exactly one arcsecond (one 3600th of a degree) sits one parsec away. Astronomers stitched 'par' from parallax to 'sec' from arcsecond and got a word that quietly records its own measuring trick.