Functional programming
Paradigm where computation is expressed as composition of pure functions — no mutable state, no side effects, values flow through transformations.
Core ideas: pure functions, first-class functions, immutability, recursion instead of loops, higher-order functions (map, filter, reduce). Statically typed FP languages (Haskell, OCaml, Elm) draw the line strictly; dynamic ones (Clojure, Elixir) are more pragmatic.
Influence is everywhere: lambdas in Java, LINQ in C#, Stream in JS, React hooks, Rust's Iterator. The "functional core, imperative shell" pattern dominates modern architecture even in non-FP languages.