Dressy casual (or "dressy/cas") is a dress code that's somewhere in-between Formal and Casual. I think it's pretty much the standard for non-wedding invitations.
I hate it because it's so imprecise. Okay, a tuxedo is probably too formal and swim trunks too casual. But what about jeans? Shorts? Sandals? I've seen other guys wear them to dressy casual parties and events; but I also see guys in Sport Coats and Penny Loafers. It's all very confusing for me, especially since I'm fashion challenged. I think it's just best to wear what you like and someone doesn't like it then tough.