REPL Read-Eval-Print Loop
An interactive shell where you write code one line at a time and see the result immediately.
Type python in the terminal and you're in a REPL. It takes a line (Read), runs it (Eval), prints the result (Print), and loops back. Good for experimentation, learning, quick checks.
Most modern languages have a REPL: Python, Ruby, Node.js, PHP, Rust, Swift. Lisp invented the concept in the 60s.