Nomenclature is a big, complicated sounding word for a simple concept- the naming of things, such as species of animals.
Some people don't take naming as seriously as they should, calling things whatever they like regardless of that thing's official designation, which is just plain wrong.
Because, as this comic from Mr. Lovenstein shows, it's important to call a tortoise a tortoise, or you might make it shella angry.
-Via Geeks Are Sexy