A Bessel function is a type of mathematical function that appears in various problems involving wave propagation, heat conduction, and vibrations. Named after the mathematician Friedrich Bessel, these functions are solutions to the Bessel differential equation, which is a second-order linear differential equation. They are particularly useful in cylindrical or spherical coordinate systems. Bessel functions come in different types, primarily Bessel functions of the first kind and Bessel functions of the second kind. The first kind is finite at the origin, while the second kind diverges. These functions are widely used in engineering, physics, and applied mathematics to model physical phenomena.