A Functional Language is a programming language built over and around logical functions or procedures within its programming structure. It is based on and is similar to mathematical functions in its program flow.
Functional languages derive their basic structure from the mathematical framework of Lambda calculus and combinatory logic. Erlang, Lisp, Haskell and Scala are the most well-known functional languages.
Source: https://www.techopedia.com/definition/19505/functional-language