time it takes the earth to revolve around the sun
A 2000-year time period is known as a millennium.
A century
lustrum
The period of time that the school year is divided into is called a "term." Each term typically consists of several weeks or months during which students attend classes and complete coursework.
fiscal year
It COULD be called a Gigayear, but in practice it is simply called "a billion years".
A period of one year can be called a year, or astronomically speaking, a revolution.
It is called the planet's period.
It is called a year because it takes that much time to complete one orbit of the sun.
The time a planet takes to revolve around the sun is called its orbital period.
A time a person rules is called a 'Ruling Period' or 'Reign'
A ten year period is a decade.