1The amount by which one number or magnitude exceeds another; = "difference". Contrasted with geometrical ratio.
2A manner or mode of change in which successive quantities form an arithmetical progression; an arithmetical progression.
Used especially with allusion to Thomas Malthus and his work on the rate of increase of populations.
Early 18th century; earliest use found in Alexander Malcolm (1685–1763). From arithmetical + ratio, after post-classical Latin arithmetica ratio.