PL/I (pronounced as if it were written 'P L 1'; some people write it as PL/1, but this is technically not accurate) is a programming language that was initially developed in the 1960's. Produced by IBM, it can very roughly be thought of as ALGOL combined with COBOL, and then further 'enhanced' (i.e. complexified). (This is because it was intended as a 'universal' programming language; i.e. one suitable for use in all problem domains.)
Although little-used nowadays, it is historically somewhat important as the language used by the Multics project.