That which is very fashionable or trendy to do. When used in negative constructions, "in" is often emphasized in speech or put in quotation marks in writing. When it comes to weddings, the in thing to do these days is have a smaller ceremony in an exotic location. I know it's not the "in" thing to do for a lot of people, but I think staying home with the kids instead of continuing to work was the best decision for me and my family.