Infinitesimal
Newton and Leibniz developed the calculus based on an intuitive notion of an infinitesimal. In 1870, Karl Weierstraß provided the first rigorous treatment of the calculus, using the limit method. But in 1960, Abraham Robinson found that infinitesimals also provide a rigorous basis for the calculus. Many calculus courses still begin with the difficult limit concept, but others now adopt the easier to understand infinitesimal concept.