When you're flirtatious, you have a playful kind of banter, especially with someone in whom you're romantically interested. Your heart will flutter if your crush gives you a flirtatious smile.
You might expect two lovesick teenagers to be flirtatious with each other, while it's deeply embarrassing to watch your grandmother treat her favorite waiter in a flirtatious way. Don't take it too seriously, though — there's a lightheartedness to all flirtatious behavior. In fact, the unserious 16th-century meaning of flirt was "to flit inconstantly from object to object."