Stefan Banach (born March 30, 1892 in Cracow, died August 31, 1945 in Lviv) was a Polish mathematician, one of the representatives of the Lviv school of mathematics. He was the creator of functional analysis, a large and important branch of mathematics in modern applications, which helped to find equations used to describe complex natural phenomena, including quantum mechanics.

Member of the Polish Academy of Learning (1924), member-correspondent of the Academy of Sciences of the Ukrainian SSR (1940).

Banach's first works dealt with Fourier series (in the first paper published jointly with Steinhaus he negatively solved the problem of convergence of the mean of partial sums of Fourier series), orthogonal functions and series, Maxwell's equations, derivative functions, measurable functions, measure theory. In his doctoral thesis (published in 1922) and in the monograph Théorie des opérations linéaires gave the first axiomatic definition of spaces, later named after him (Banach spaces), which he himself defined as Type B spaces. He finally established the foundations of functional analysis, which is extremely important in modern applications of mathematics. He gave its fundamental theorems and introduced its terminology, which has been accepted by mathematicians all over the world.