A half-marathon is a road race that covers a distance of 13.1 miles or 21.0975 kilometers. While the term half-marathon is by far the most common, there are a few synonyms that can be used to describe this type of race. One such synonym is a 21K, which is short for 21 kilometers and is often used outside of North America. Another synonym is a 13.1 miler, which is a colloquial term used to describe a half-marathon in miles. Additionally, some races may use the term mini-marathon or 10K+ to describe a half-marathon distance.