Popup is written in different ways: popup, pop up, and pop-up. It can be a touchy topic. Some people write it as 'popup' and others 'pop-up'. Neither is wrong.
On the other hand, 'pop up' is the verb that describes the action of popping up. The difference between 'popup' and 'pop up' is that popup is the tool and pop up is the action that it performs.
But the difference between 'popup' and 'pop-up' depends on who you ask. Some might say the correct term is pop-up, but it's commonly spelled as popup for SEO purposes. Technically, since both these terms are widely used, neither 'popup' nor 'pop-up' is wrong.
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