Why is the Catholic Church called "catholic"?

The word 'catholic' comes from the Greek 'katholikos,' meaning universal or according to the whole. The Church is called Catholic because Christ sent her to all peoples of every time and place, and because she professes the fullness of the faith, sacraments, and means of salvation — not a partial or regional gospel, but the whole of divine revelation.

Fullness of Faith and Means of Salvation

Those who are baptized share in this catholicity and bear a responsibility to carry the whole gospel to the whole world. The Church's catholicity is thus both a gift received and a mission entrusted.