Pecans, cashews and almonds are not nuts at least according to the botanical definition, which is a a dry fruit in which the ovary wall becomes increasing hard as it matures and where the seed remains free within the ovary wall.
What they are, are drupes, which is defined as having fleshy part surrounding a pit with a seed inside. Think peach. Peaches are drupes.
This is very interesting. I had no idea. If you open a peach pit, the seed inside looks like an almond!
Mulberries are drupes, too, if I remember correctly.