What is a 'flip' in Yu Gi Oh?

Answer 1

A 'flip' happens when a monster goes from being face-down to face-up. This should not be confused with a Flip Summon which is a more specific kind of flip. So a flip can happen when.

1 - A player manually changes the monster from face-down defence to face-up attack, this is called a Flip Summon and is also a flip.

2 - A face-down monster will flip when attacked.

3 - Some card effects can flip monsters. Swords of Revealing Light is one.

Note that some things trigger from being flipped, and some trigger from being Flip Summoned. The former will trigger from any of the above, the latter triggers only from the first, it will activate if attacked or flipped by an effect.