What the heck is "casual", anyway?

So, I'm going to an Event Friday night, as a chauffeur and companion for my father. Said event's invitation says "Casual Attire". And I'm going through wide swings in my guesses as to what to wear.

The event is a retirement dinner for a good friend of my father's, a man who is a fairly high-up State employee in the environmental area. The bureaucrat role makes me think "dress up, people will be in business suits", though the environmentalist role edges the scale down some. I think. The dinner will be held at the Southface Institute, offices of a group that promotes environment-friendly housing.

My work clothes are generally jeans or khaki pants, and a cotton shirt. If I'm the hostess and I say "casual", I mean your raggedy cut-offs and T-shirt are perfectly fine. Clearly, my definition of casual must be upscaled for this. :) I'm debating between a summery dress, or a slacks-and-jacket outfit of some sort. All this is complicated by my change in sizes--I don't have any nice slacks that fit right now, nor any summer jackets. I've got dresses I think are too dressy (but one of which might get worn anyway) thanks to things I hoarded for years, but nothing that seems right.

Anyone got opinions? I think I'll make one shopping trip to see if some outfit just hits me as right for this, but I'm not much good at undirected shopping.