There are a number of terms and models (only slightly different) that describe this. Personally I think you're better off simply referring to it as the natural reading order. The "shape" isn't really it's own thing, it's just a result of most Western languages reading right to left, top to bottom. We've applied this to the digital realm. In no small part thanks to Western influence in the creation of the early Web and computer technology in general, this reading order has become common even in Left to Right language websites or bilingual websites as a kind of default for layout of elements.
Vanseo Design's 3 Design Layouts offers three related terms to define this: The Gutenberg Diagram, Z-Pattern Layout and F-Pattern Layout. If you'll look, you'll realize they're really the same thing, visualized in superficially different ways. For what it's worth, more web design people will likely recognize F Pattern or F Shape but you'll get more design geek cred by calling it The Gutenberg Diagram. Again, if you're using plain English or conversing with users not designers, I'd strongly suggest referring to it as (natural) reading order.
No, reading isn't "natural" depending on your definition, but that doesn't really matter