Which is greater, a parsec or a light year?
A parsec is greater. It is approximately equal to 3.3 light years.
Parsecs are the preferred unit used by astronomers when talking about distances. A parsec is defined as the distance that an object would have to be from the sun to have a parallax angle of 1" (one arc-second). Therefore, any measurement found using parallax will yield an answer in units of parsecs. This makes the parsec a convenient standard unit for the measurement of large distances in space.
A light year on the other hand refers to the distance that light travels during one year. This gives us information, not only about how far the light has traveled, but also how long it has been traveling. Light years are more commonly used in popular science, because it makes more intuitive sense for people who are not familiar with parallax measurement.
To put each of these units into perspective;
Note that the numbers given are rounded and not exact.