Scheme
Minimalist Lisp dialect — Gerald Sussman + Guy Steele, MIT, 1975. Just a handful of primitives, everything else built on top. Famously simple + deep.
The SICP book ("Structure and Interpretation of Computer Programs") uses Scheme and was the standard MIT intro CS course for 30 years. Implementations: Racket (research dialect, its own language), Guile (GNU's embedded script), Chicken (compiles to C), Chez Scheme (fastest). R5RS (1998), R6RS (2007, controversial), R7RS (2013). Continuations + tail call optimization are the language's signatures.