In astronomy, an astronomical unit is defined as the average distance from the Sun to the Earth, or about 150 million kilometers (93 million miles). You can abbreviate astronomical unit as AU.
Since the distances in astronomy are so vast, astronomers use this measurement to bring the size of numbers down. For example, Earth is 1 au from the Sun, and Mars is 1.523 AU. That’s much easier than saying that Mars is 227,939,000 km away from the Sun.
Using astronomical units is even more helpful when the distances get larger. Pluto is 39.48 AU from the Sun, and the newly discovered Eris is 67.67 AU. The Oort Cloud, the source of long-period comets in the Solar System, is thought to be 50,000 AU from the Sun.
Here are some handy conversions for you:
We have recorded a whole episode of Astronomy Cast about measuring distances. Check it out here, Episode 10: Measuring Distance in the Universe.