Yup, not really a difference! I think ampoule traditionally comes from K-beauty, like they created the terminology. But the functionalities of serum and ampoule are the same–essentially to treat specific skin concerns.
There isn't a huge difference! Ampoules are often thicker in consistency than serums and often have a higher concentration of active ingredients to help target skin concerns.
They belong in the same step in your skincare routine – before your moisturizer and after your toner.