Biology noob here, trying to expand his basic scientific knowledge.

A unicellular organism, also known as a single-celled organism, is an
organism that consists of a single cell, unlike a multicellular organism that consists of multiple cells.
But to me, this is ambiguous. Is a unicellular organism one that contains one cell and one cell only? Or is a unicellular organism one that contains one kind of cell, but lots of copies of that cell? So for example, are plants not unicellular because they contains groups of different types of cell, or are plants not unicellular simply because plants contain more than one cell?