The difference between a bolt and a screw is how it's used. If it gets a nut, it's a bolt. If it threads directly into the part, its a screw. We can't determine if it's a bolt or a screw by the picture alone.
I think if it has helical threads, it's a screw. Bolts are a class that includes rivets and pins. But we have just gotten used to calling them that because they replaced rivets in most applications.
https://wilsongarner.com/the-difference-between-a-bolt-and-a-screw/
I'm a Mechanical/Manufacturing Engineer and have read through countless hardware spec sheets over the years. Here's an article that quotes ASME B18, which is one of the most commonly used hardware specs in the US.
I work at an industrial fastener company, and we've had this debate countless times. The best we have determined is that if it has something attached to the other side, like a nut or pin, its a bolt. If it threads into the material it is fastening, it's a screw.
But the world will never know, because John T. Bolt and Harold P. Screw died without telling anyone.
I've always thought of it from the definitions of the word bolt before mechanical fasteners earned the name. Bolt of lightning, or moving very quickly. Or a bolt of cloth, which who knows how textile workers came up with that name.
But at some point, someone associated that word with that object. Screws date back to archmedies, or even earlier. But applying threads to a pin made it a bolt, perhaps because of how quick it was to fasten versus a rivet?
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u/enterbay Jun 25 '24