The term ESS can refer to three different things. The first is 'CURVE', which is a line that bends and changes direction, often in a smooth and continuous way. The second is 'SIGMA', which is the eighteenth letter of the Greek alphabet and is often used in mathematics and science to represent standard deviation or summation. The third is 'curveinaroad', which is a phrase used to describe a road that has curves in it, making it more interesting and challenging to drive on.