Because it is a combination (a combined event) of ten individual events by the same person.
A decathlon is, in order
Day 1
100m
Long Jump
Shot Put
High Jump
400 m
Day 2
110H
Discus
Pole Vault
Javelin
1500m
Read more: What_are_the_olympic_field_events_in_the_decathlon
Chat with our AI personalities
Because it uses the root hepta which means 7. thlon means some type of competition. "7 event competition".
Derived from the Greek words deka (meaning ten) and athlos (meaning contest) [Decathlon is a contest of ten events]