In the strictest interpretation of the question, a coordinated turn occurs as posted above when the drink on the table doesn't slosh in the least bit.

In a true coordinated turn the normal acceleration on the plane is straight down through the bottom of the plane as if you were flying straight and level.

That may OR MAY not require rudder to achieve on any given airplane. And unless you mount a rate of turn indicator in your plane and either videotape it or other wise record what it's doing in a turn there is absolutely no way to determine if you are in a true coordinated turn from the ground!

Adding rudder for the sake of adding rudder because someone said that makes it a coordinated turn is just wrong.

One test you can perform to test if you need to add rudder is fly the plane over your head and away from you. Now do an aileron turn in one direction. Note if the nose of the airplane goes the wrong way for a moment before it turns in the direction you've directed. If it does go the wrong way then you have a case of adverse yaw, and you should add a little rudder to start the turn. Now there is the possibility that the nose will go too far in the direction of the turn before the plane actually starts turning. Then you have proverse yaw and you actually need OPPOSITE rudder to make a coordinated turn. This is also the situation when you add rudder for the sake of adding pro turn rudder when you don't actually need it.