In the original archaic definition the term ‘Republic’ it could be applied to literally any form of Government that wasn’t overtly tyrannical. Monarchy’s could and were considered Republics under the oldest definition.

However in modern day. The conflation of ‘Democracy’ and ‘Republicanism’ at least in regards to the U.S started with James Madison. Who conflated the idea of Representative Democracy with Republicanism. As we technically do meet the bare minimum definition for both.

Since then the terms grew to be used interchangeably. With ‘Representative Democracy’ eventually being shortened to Democracy. And World War 1 and 2 eventually making it fashionable to throw around the term Democracy more often than ‘Republic’. It’s only really been since about the later 20th century. That people have once again sought to establish a dividing line between what differentiates a Republic from a Democracy. Whether it be representative or some other form.