Not just a word Spock uses to sound cool.

A parsec is an actual unit of distance. The name is derived from two words: parallax and arcsecond. A parsec is the distance an object is from Earth if it's parallax is one arcsecond.

Of course, this has very little meaning to anyone who is not an astrophysicist.

A parsec is equal to 3.086*10^18 cm, 3.26 light-years, or 206265Astronomical Units. In other words, it's a freaky big distance. Here's an idea of how big:

The distance to closest star (other than the Sun) is about 1 parsec.

The distance to the center of the Milky Way is about 8,000 parsecs.

The distance to the Andromeda galaxy is about 1 million parsecs.

The distance across the entire universe is thought to be several billions of parsecs.