Civilization is something that is not easily defined. It is not easy to identify the exact moment at which a group of people becomes a civilization. However, we can give a broad definition of civilization, even if it is hard to apply it to specific cases.
In general, a civilization is a group of people who have come together and formed a political unit centered on a dense population. The dense population generally lives in a city that is the center of the civilization (this is an early civilization we are talking about; modern civilizations do not have to have one single city that is their center). The city typically has a governmental function and a religious function.
Civilizations have to have economies that are based on farming. They cannot be made up of hunters and gatherers. Agriculture allows people to produce surpluses of food. This allows them to feed people like government officials, priests, artisans, and others who do not farm. Agriculture, therefore, allows a population to have the governmental and religious organization and the diversified economy that is necessary for a civilization.
In short, then, a civilization must have a dense population whose economy is based on agriculture but has diversified. It must have a complex society with a government and a religion. These criteria must be met in order for a society to be called a civilization. (Again, this is referring to a time when civilizations were first appearing. Our modern civilizations do not typically fit all of these criteria.)
Civilization is the stage of human social development and organization that is considered most advanced. According to Merriam-Webster, this is the condition that exists when people have developed effective ways of organizing a society and care about art. It is also defined as a particular well-organized and developed society.