World

A world (formerly level) is an individual Minecraft "universe" which encompasses an instance each of the Overworld, the Nether and the End. Worlds can be accessed through the world selection screen from the main menu and can be created with the "Create New World" button.

Properties
Worlds are split into three dimensions, which are in turn composed of blocks of various sizes, usually either cubic or composed of one or more cuboids, or of flat plane images, all of which are arranged in a Euclidean grid. Fluids, entities and particles also exist within these worlds, and certain locations of the world can possess biomes and different levels of light. Blocks and entities can possess other attributes such as items, status effects and, by extension, enchantments. Various structures can also generate inside worlds (provided that the 'Generate structures' world option is turned on), sometimes containing chests with loot inside.

Java Edition
Each world is saved in its own separate folder in the .minecraft/saves folder. Files relating to chunks in the Overworld are stored in the base world save folder, Nether files are stored in the DIM-1 subfolder, and End files are stored in DIM1. Each dimension has its own folders that is stored in its directory.

Bedrock Edition
Each world is saved in its own separate folder in the folder. All chunks related to all dimensions are stored in the  folder.