By pinafore dresses, what I generally mean is a dress without sleeves which is (probably) designed to be worn with something underneath, for example a T shirt or jumper. I have to say that, as I am British, for me a jumper is a knitted top with long sleeves - what Americans may call a sweater. I'm aware that in the US, "jumper" seems also to be another name for a pinafore dress! And the patterns I'll be covering here include some that are called jumpers. I will stick to calling them pinafore dresses, to avoid confusion for my British readers. (One day I'm going to do a post on ambiguous cross-Atlantic terminology in sewing clothes!)
