The term RHO-TAU LINKUP is a five-letter word, SIGMA, which is used in mathematics and science to represent the sum of a series. It is also used in engineering and physics to denote the ratio of two forces. In addition, it is used in chemistry to denote the ratio of two elements in a compound. SIGMA is also used in statistics to denote the standard deviation of a population. Finally, SIGMA is used in computer science to denote the number of states in a finite state machine.