Here's why:

* Eukaryotic cells are characterized by having a membrane-bound nucleus that contains their genetic material (DNA). This distinguishes them from prokaryotic cells, which lack a nucleus and have their DNA free-floating in the cytoplasm.

* Single-celled eukaryotes are called protists. Examples include amoebas, paramecia, and algae.

* Multicellular eukaryotes include all plants, animals, and fungi.

So, any organism whose cells have a distinct membrane-bound nucleus is considered a eukaryote, regardless of whether it's single-celled or multicellular.