Is there a difference between imidazolidinone and imidazolidone? I've had my nose in the early literature on GFP (Green Fluorescent Protein), which contains a 4-(p-hydroxybenzylidene)imidazolidin-5-one fluorophore, and in one paper the author refers to the "imidazolidone ring".

I assumed that this change of spelling was equivalent, but I can't find anything that looks reliable on Google (there are also pages referring to imidazolinone, which looks like it refers to an aromatic ring with Nitrogens at positions 2 and 4...)

I don't think I'd know quite where to look on the IUPAC website for the info so trying here...

Can anyone help me understand the variations?
According to the NIST ChemBook, 2-imidazolidinone and 2-imidazolidone are synonymous. The official IUPAC name is imidazolidin-2-one. 1,3-ethyleneurea is also another name for this same molecule:

    



But, as you note, tiny changes in spelling can sometimes be much more tricky than this. To take an example from this family: imidazolidinone and imidazolinone (note that the later ends in –inone, not –idone) are ketones formed from imidazolidine and imidazoline, respectively. Imidazolidine and imidazoline are partly saturated and fully saturated imidazole derivatives:

   

Imidazolidine is a proper IUPAC name, but the official name of imidazoline is 4,5-dihydro-1⁠H-imidazole.