What does the empty chair at the Seder Meal represent?

There is no establish tradition of having an empty chair at a seder. There are some groups today who have an empty chair at the table in honour of friends and/or family who have been killed in war or who are in the military and cannot be present at the seder.

The actual tradition is to have a full glass of wine on the table during the seder. This cup is for the prophet Eliyahu who is said will arrive before HaMoshiach does to anounce his arrival and anoint him.