I started from a joke, towards the beginning of A Wrinkle in Time Calvin is quizzing Meg on random things, because everyone at school knows she's dumb, and her mom says "no, actually she's brilliant at math and science and unengaged on everything else" and it's one of the questions.

I have turned to Wikipedia. A megaparsec is a million parsecs, about 3,260,000 parsec. Astronomers use it for the space between galaxies.