The term 'sigmoid curve' is a mathematical concept that describes a smooth, S-shaped curve. It is also known as an S-curve or sigmoidal curve. The term 'ESS' is an abbreviation for the word 'ESSENCE, ' which is defined as the most important or essential part of something. In this context, the ESSENCE of a sigmoid curve is its smooth, S-shaped shape. This type of curve is often used in mathematics and statistics to describe the relationship between two variables. It is also used in economics to describe the relationship between supply and demand. Additionally, it is used in biology to describe the growth of populations over time.