Definition:Epicycle of Hypocycloid/Also known as

Hypocycle of Hypocycloid: Also known as
The hypocycle of a hypocycloid is also known as its generating circle.

Some sources fail to name it, and instead refer to the small circle.

It can also be referred to as a rotor.

The term has been taken by from the children's toy, which itself was taken from the term  as applied to the moving (rotating) part of a rotary machine.