For questions about infinitesimals, both in an intuitive sense as well as more rigorous settings (see also [nonstandard-analysis]).

Intuitively, an infinitesimal is an infinitely small number. They played a significant role in both Leibniz's and Newton developments of calculus, as well as in the work of Archimedes.

For a more rigorous treatment of infinitesimals, also see nonstandard-analysis.