Definition of "what's up" in English

Phrase

1. What’s the matter? What is the problem? What is wrong?

   • You're looking a bit miserable. What's up?
   • I got a call saying to come straight away. What's up?

2. What's happening? What are you doing?

   • Hey, what's up? – Not much, just working.

3. A casual greeting with a meaning similar to hi or nice to meet you.

   • Hey, what's up? – Hey! Good to see you.