Periodization is the tendency to have distinct periods of history or time that don’t overlap. So sayeth wikipedia. Everything within a period will fall under certain scientifically or culturally defined criteria. They also have a definite start and end, even if the defining is only done by another period starting or ending for some reason. The following is a list of useful terms you can adapt to your world or timeline for your historical use. These are in order of relativity. (Do keep in mind that every culture will and should define these periods of time differently. The definitions I have listed have a western bias, so adjust as you see fit.) Aeon- Often confused with Eon, this is a very long, indeterminate amount of time. Often used to represent simply “A very long time” Super Eon- The largest period in a geological time scale. Composed of Eons. Often defined by mineral strata or star stage. Eon- Smaller than a super eon, larger than an era. Usually defined by rocks and major lifeform evolutions. Era- Smaller than an eon, larger than a period. Defined by lifeform evolution and fossils or art/craft/science development. Period- Smaller than an era, larger than an epoch. Defined by moderate evolutionary changes in life, music, art, or craft (or science or society). Epoch- Smaller than a period, larger than an age. Defined by life or society. Age- The smallest geologic and time marker for large periods of time. Also the last of the relative scale. The least or most abrupt change happens. Millennium- One thousand years. Century- One hundred years. Decade-Ten years. Year- One rotation around the planet in question’s sun. Also referred to as Summers, Winters, Passes, and Rotations. Other time periods within history and within a year might be subjective and involve that planet’s (and culture’s) unique history, religion, and surroundings.