The term 'Rho Follower' is a crossword solution that can be filled in with the word 'SIGMA, ' which is a five-letter Greek letter. SIGMA is the eighteenth letter of the Greek alphabet and is often used in mathematics and science to represent various concepts, such as standard deviation and summation. It is also used in chemistry to represent the number of atoms in a molecule. In addition, SIGMA is used in many other fields, including engineering, economics, and linguistics.