Sigmoid curves are a type of mathematical curve that is shaped like an 'S' and can be used to model a variety of phenomena. The term 'SIGMOID' comes from the Greek letter sigma, which is the shape of the curve. The two crossword solutions, ESSES (5 letters) and OGEES (5 letters), are both related to SIGMOID curves. ESSES stands for 'exponential SIGMOID, ' which is a type of SIGMOID curve that is used to model exponential growth or decay. OGEES stands for 'odd-symmetric generalized exponential SIGMOID, ' which is a more complex type of SIGMOID curve that is used to model more complex phenomena.