These are neither formal definitions – to provide such a thing would be to over-simplify an inherently complex situation – nor are they ‘fixed’, as they will change as the context changes.
Nid diffiniadau ffurfiol yw'r rhain – byddai hynny'n gorsymleiddio sefyllfa gynhenid gymhleth – ac nid ydynt yn 'sefydlog' chwaith, gan y byddant yn newid wrth i'r cyd-destun newid.