A pop-up window is a small, new window that suddenly appears (pops up) on your computer screen, typically over the current webpage or application, to display ads, notifications, forms (like newsletter sign-ups), or extra content, often interrupting the user's main task and sometimes requiring a click to close or interact with. While once popular for marketing, they are now often blocked by browsers due to their intrusive nature, leading to less disruptive alternatives like modals or in-page notifications.