The term "bottom side" is a colloquial expression used to describe the underside or reverse side of an object. It can refer to either the front or back, depending on the context in which it's being used. For example, if you are talking about the bottom side of a piece of paper, it would mean the part that is touching the surface it is placed on - whether it be the front (top) side or the back (bottom) side. Similarly, when referring to an object with two sides, such as a coin, "bottom side" would indicate the reverse side facing down when lying flat.