Why are Russian space travellers called cosmonauts?

Cosmonauts are people certified by the Russian Space Agency to work in space. Derived from Greek word "kosmos", meaning "universe", and "nautes", meaning "sailor", the term was officially recognised after Soviet's Yuri Gagarin became the first human in space in 1961. However, three weeks later, Soviet's Cold War rivals, the US used the term 'Astronaut' for first American in space.