The answer to the crossword puzzle is OPER, which is a four-letter word. It is an abbreviation for the phrase 'it's to the left of #'. This phrase is often used in mathematics and other sciences to indicate that a number or symbol is located to the left of another number or symbol. For example, if someone were to say '2 is to the left of #', they would be referring to the number 2 being located to the left of the symbol #.