Gridiron football is a term that refers to both American football and Canadian football. The term came about after it was observed that the field of play in each sport, because of the many lines marking it, resembles a gridiron.
The word "gridiron" alone may refer either to the field or to the sport; however, in North America it's usually only used to refer the field. In some other English-speaking countries--particularly Australia and New Zealand--it is the primary term used to refer to the sport, differentiating it from other forms of football such as Australian rules football, association football (soccer), or rugby football.