1 : a running race that is about 26 miles (42 kilometers) long
He ran (in) a marathon.
2 a : something (such as an event or activity) that lasts an extremely long time or that requires great effort
We watched a marathon of our favorite movies.
a movie marathon
a shopping marathon
2 b : a contest in which people compete with each other to see who can do something for the longest amount of time
a dance marathon