Stomach, gaster

How do you English say when talking unprofessionally and not health-related: The cat lies on my belly or on my stomach? How do you call gaster (organ) in everyday life? And when health-related: Bladder or liver is in the stomach? (I know for abdomen) And do you say: gaster lies in the stomach?

I am from the U.S. We said, "I have butterflies in my stomach," meaning one is nervous. People say "stomach" when they really mean abdomen. Is that what you're asking?

Yes, I see: stomach = abdomen. How do you call an organ which is called gaster in latin and lies between the esophagus and the duodenum?

I don't know what a "gaster" is related to humans.

See 1c Merriam-Webster about stomach = abdomen.