The term 'Period in Cosmology' can be defined as an EPOCH, which is a unit of time in cosmology. An EPOCH is a period of time that is marked by specific events or changes in the universe. It is used to measure the age of the universe and is typically measured in billions of years. The term is often used to describe the different stages of the universe's evolution, such as the Big Bang, the formation of galaxies, and the current state of the universe.