A parsec is defined as the distance from the Sun which would result in a parallax of 1 second of arc as seen from Earth.

Since distances of nearby objects can be determined directly using parallax observations combined with elementary geometry, the parsec was historically used to express the distances of astronomical objects from the Earth. This tendency has been carried over to some extent so that distances to objects are still often expressed in units of parsecs instead of light-years.

Light-Year