This essay examines the definition and early development of the term “Little Ice Age,” a source of controversy in early modern studies and historical climatology. American geologist François Matthes first used the term “little ice-age” to describe the preceding 4,000 years, during which glaciers reached their greatest extent since the final Ice Age of the Pleistocene. Matthes’s reports for the American Geophysical Union’s Committee on Glaciers, however, demonstrate the critical importance of the sixteenth through nineteenth century. This essay discusses the relationship between Matthes’s theories of glaciation and climatic periodization and those of his predecessors and contemporaries, including Charles Rabot, Gordon Manley and G.S. Callendar. This essay also examines the early adoption and diffusion of Matthes’s evocative term, which appeared in popular publications only a decade after its initial introduction.