Cardboard refers to a stiff, durable material made from a paper pulp. This material is widely used across the world for various packaging purposes. There are several synonyms that one can use to refer to this material. One of such synonyms is "carton," which is a term commonly used to refer to boxes made from cardboard. Another synonym for cardboard is "corrugated fiberboard," which is a type of cardboard that has a corrugated layer sandwiched between two flat layers of cardboard. Additionally, other synonyms that can be used to refer to cardboard are "box," "container," or "packaging material." All these terms can effectively be used to describe different types of cardboard materials that are available in the market.