SML, or Standard ML, is a functional programming language known for its strong type system and type inference capabilities. It was designed in the 1970s and is widely used in academic settings for teaching programming concepts and language design. SML supports features like pattern matching, higher-order functions, and modules, making it a powerful tool for developers. One of the key aspects of SML is its emphasis on immutability, which means that once a value is assigned, it cannot be changed. This characteristic helps prevent certain types of errors and makes reasoning about code easier. SML is also part of the ML family of languages, which includes others like OCaml and F#.