I read somewhere about 15 years ago that the term was introduced when laptops would get so hot that the legal departments at laptop vendors started to worry about their liability if they kept calling them laptops and they got sued by someone whose lap got burned.
Now that Apple's laptops run much cooler, my guess is that they continue to say "notebook" because Apple owns an important trademark that ends in "Book".
This is a bugbear of mine. I've never met a real human being who uses that term. I don't even know if people would understand me if I started using it in normal conversation.
They are laptops outside of industry marketing material. Why can't they just admit they lost this rebranding battle?
