What's the difference between "single-celled organisms" and "unicellular organisms"

They are both organisms that consist of only 1 cell.

However unicellular organisms can be classified into eukaryotic organisms such as the Amoeba where single cellular organisms is classified into prokaryotic organisms such as bacteria.

Prokaryotic organisms(single-cellular) undergo binary fission wheres Eukaryotic organisms(Unicellular) undergo multiple binary fission.

Nuclei/Nucleus refers to one of the main organelles in a cell which contains all the genetic data. DNA

A unicellular organism also known as a single organism is an organism that consists of only one cell, in contrast to a multicellular organism that consists of multiple cells.

Single and uni are same!!