From Vikidia, the encyclopedia for 8 to 13-year-old children that everybody can make better
An atlas is a collection of maps. It is a common tool in geography but also in several other disciplines considering where things happen: history, economy...
Today we usually think of an atlas as being a book comprising essentially maps illustrating a common theme, such as the Earth, a given country, migrations, economic flows, possibly another planet... Abraham Ortelius edited the first modern atlas that really corresponds to this definition: first printed in 1570, its Theatrum Orbis Terrarum ("Theatre of the World") is a book of geographical maps of the World, of the same size and style (although drawn by several people), organized by continents and regions, accompanied with explanatory text.
In the past and in a more general sense, an atlas is a collection of maps that may be very disparate (created by various people but also in various contexts, in various styles...) and be compiled over a long period of time, possibly complemented with pictures and drawings of animals and plants, buildings, people and even mythological events.