The crossword solutions for the term SUBSTITUTE: ABBR. are ALT (3 letters) and REPL (4 letters). The abbreviation ALT stands for 'ALTERNATE' and is used to refer to a person or thing that can be used in place of another. REPL stands for 'REPLACE' and is used to describe the act of replacing one thing with another. Both terms are commonly used when referring to a substitute, which is someone or something that takes the place of another.