Lavatories

So I was sitting on the plane awhile back listening to the whole schpiel they do at the beginning. And it hit me... Why is it that a bathroom is only called a lavatory on a plane? We all accept it... but I'm actually going to challenge it.

I've never heard the use of that word except for on a plane, but for purposes of convenience and mainstreaming. Let's simplify here... there are some already common phrases out there to choose from: bathroom, restroom, or even toilet is a more common phrase. Anyone else feel this way?