Each Minecraft dimension is just an ever-expanding collection of chunks.
A chunk is just a funny name for the units of land that generate as a Minecraft player explores the Overworld, Nether, or the End. Each chunk is determined by the map seed.
As seen in the image above, chunks commonly have errors that lead to a gaping hole in a Minecraft world. These holes will lead all the way down into the void. However, it is not possible for players to fall into these holes. Instead, they will just get stuck. Players who get stuck in a missing chunk will have to close and relaunch their client.
What are chunks in Minecraft exactly?
As stated above, chunks are sections of land that will continuously be generated as the Minecraft player explores their map. Each chunk is determined by the map seed, which is why identical maps can be played by different single-player players.
More specifically, a chunk is a plot of land that is 16 blocks long, 16 blocks wide, and 256 blocks tall, which reaches down to the bedrock. In the upcoming Minecraft 1.18 Caves and Cliffs update, a chunk will be 384 blocks tall as the world height will increase.
A chunk will be applied to different properties depending on its loading priority.
As stated above, chunks will load certain blocks or entities depending on their loading priority. Priority is determined by the ticket, which is granted when the player is in a chunk.
A chunk that a player resides in is considered a "player ticket chunk." The chunks around the player's ticket chunk will all load gameplay aspects differently.
For example, all gameplay features are active in the ticket chunk. However, certain chunks that are farther away from the ticket chunk will prevent redstone activation from occurring until the player is close enough to it.
A ticket can also be forced with commands. These tickets will be loaded indefinitely. This can be useful if the player wants a farm to continue working even when they are not near it.
The video above provides a visual guide to chunks in Minecraft.