The term PI-SIGMA GO-BETWEEN is composed of three parts: PI-SIGMA, GO-BETWEEN, and RHO (3 letters). The solution to the crossword for this term is RHO. PI-SIGMA is a Greek letter combination that stands for the phrase 'Peace and Security'. GO-BETWEEN is a term used to describe someone who acts as an intermediary BETWEEN two parties. RHO is another Greek letter which is often used to represent the number 100. Therefore, the complete term PI-SIGMA GO-BETWEEN can be interpreted as 'Peace and Security through 100 intermediaries'.