"...  It's a simple question ..."  Is it?  Yet I notice that you don't answer it ... :-/

Indeed, web search of NASA, NOAA, or JPL on a quick definition reveals these organizations to be remarkably opaque about what constitutes this major metric of modern climate crisis science.  A NOAA page comes closest when it admits that the notion of a 'global average temperature' must seem next to meaningless to the nonspecialist when the Earth's surface on any given day exhibits a temperature range of ~ 100 deg. F. - but then they drop the topic like a hot potato and never answer the question.

What's worse, I've hear Ph.D. physicists wryly observe that climate scientists tend to talk of temperature like it's some amount of tangible 'stuff" (instead of a thermodynamic state function), and that a 'global mean temperature' is about a scientifically useful as a 'global mean telephone number'.

I expect that climate scientists don't like that characterization very much.

But to return to the ...