Definition:Epicycle of Hypocycloid/Also known as

Epicycle of Hypocycloid: Also known as
The epicycle of a hypocycloid is also known as its generating circle.

Some sources call it the hypocycle, but (while logically consistent with the meaning of hypocycloid) this is rare.

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.