(American) a small, usually-flat cake-like dessert that is baked and may have a crispy or chewy texture;
e.g. I ate a cookie for dessert after lunch.
(noun)
(computing) a small file or piece of data stored on a user's computer that can be read by a website's server, by which the user can be identified and their activity on the website tracked;
e.g. The website stores a cookie in your computer by which it remembers you when you visit the site multiple times.
(noun)
(informal) a particular kind of person;
e.g. He's a tough cookie and he won't give up on this challenge.