A single-breasted jacket or suit has one line of buttons down the front, and the two sides of the coat meet in a simple overlap. Most ordinary office suits look like this. The opposite is double-breasted, with two lines of buttons and more cloth across the chest. Though actually, “breast” here is an old clothes word for the front of a jacket, not a lesson about the body. It is a tailor’s word, a bit specialised, but it is the word a shop will use.