The confusion is in part perpetuated by Apple, who can't seem to make up their mind on the matter.
My UK MacBook Pro keyboard has "alt" and doesn't mention "option" at all. They have also shipped keyboards that just say option (no mention of alt). Many of their keyboards have both printed on them.
In most of their user-facing documentation it is referred to as the "Option" key. In Cocoa, it is the "Alternate" key.
The menus don't mention either: they use the ⌥ symbol. Which is on some, but not all of their keyboards.
My UK MacBook Pro keyboard has "alt" and doesn't mention "option" at all. They have also shipped keyboards that just say option (no mention of alt). Many of their keyboards have both printed on them.
In most of their user-facing documentation it is referred to as the "Option" key. In Cocoa, it is the "Alternate" key.
The menus don't mention either: they use the ⌥ symbol. Which is on some, but not all of their keyboards.