Sigmoid

A sigmoid curve is a graph that describes the change in a parameter depending on the change in another parameter. It gets its name from the Latin letter S, which looks like an inverted horseshoe or S-shaped curve. The concept and term were introduced into astrophysics by Australian physicist George Clement Rayleigh to refer to changes in the brightness of stars over time. Since then, the sigmoid curve has become widely used in other sciences.

Sigmoid curves are S-shaped, meaning that they first rise up and then fall down, like the letter S. This is because some systems and processes go through a stage of accumulating changes before showing signs of improvement or deterioration. In other words, these curves illustrate the sequence of events that occur in a situation where the first stage takes longer than subsequent ones.