‘Atheism’ is a term that has historically carried a wide range of meanings and connotations. Popular speech, in particular, admits of a range of definitions, but the same is true of contemporary scholarly usage also. This chapter therefore surveys the sheer variety of ways of defining ‘atheism’, before outlining the pressing need for a generally agreed-upon usage in the growing—and, thus far, Babel-like—field of scholarship on atheism. It then outlines and explains the precise definition used throughout the Handbook: an absence of belief in the existence of a God or gods. The utility of such a broad definition, taking atheism to be an ‘umbrella concept’ that admits of a range of subdivisions (e.g., ‘positive’ and ‘negative’), is then explored and defended at length.