Minecraft chunk format
Minecraft chunk format is a data format used in the Minecraft game, as part of level formats such as the Minecraft Region format and the Minecraft Anvil format. Minecraft "chunks" are, at a high level, 16x256x16 (16x128x16 before the Anvil format was adopted) groups of blocks (voxels) into which the game world is split in storage and some game functionality. Storing chunks is the largest task of any overall Minecraft savegame storage system. "Chunk format", as opposed to a "chunk" in the generic sense, is here used to refer to a specific implementation of chunk storage as a structure within NBT files; this is to the exclusion of the Bedrock Edition format, etc. which store data in terms of chunks but do it in different ways.
Chunks can take up the entire NBT file, in the case of the Alpha format (in which case, whether the chunk "is in" the file or "is" the file is only a question of wording); or they may be 1024 (in a geometric group of 32*32 called a "region") to a file, in the case of the Anvil and Region formats. Their overall internal structure is largely the same between the three, except that the Anvil format stores per-block data (which includes actual block type as well as e.g. lighting) in 16*16*16 "sections". Whether split up into NBTs individually or by regions, these are then stored in the filesystem proper (within a system of subdirectories, in the case of the Alpha format).