as the headline implies, I would like to hear whether there is a definition on what constitutes as large format ?

I have been told (but I have often been told wrong...) that all sizes bigger than negatives larger (wider) than an 120 film is LF...

I don't know.
Is 6x17 then not LF?

is the classic polaroid 3.25 x 4.25" (ex in pol 665) LF?

I have seen an old reprint of a sales brochure on cameras. Here all sizes up to 8x10 was called small format (!!) - from 10x12 to 20x24 "medium format" (!!!) and above that LF....

Does the "definition" change over time?