plumbing plastic object name?

My washing machine is connected to the sink. When it had to be repaired and reinstalled, my landlord took out a round plastic object that used to be located at the site where the washing machine drain hose and the sink drain connect. He took the little piece out and threw it away. I regret not asking why he was doing that…The problem now is that the water from the sink runs down into the washing machine. This makes me think that the little round plastic thing (which has a plastic circle in the middle) was there to prevent one way water flow from sink into washing machine, while allowing the washing machine to drain through it. What is the name of this object? I need to get one and have no clue! What’s worse, I’m living in France, so I’ll need to find the translation.
Here’s what it looks like: http://imageshack.us/photo/my-images/217/plasticobject.jpg/

Thanks for your help!

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