Demi sec is a French culinary term that translates to "half dry".
Demi sec is a reference to an amount of liquid, usually used in saucemaking. A liquid added to a hot pan will evaporate. Demi sec is when the liquid has been reduced by about half.
This term is also used to describe wines. Demi sec is used to describe a sweeter wine, as opposed to the least sweet brut wines.