Back Command

Usage: /back

Teleports you to the last place you were at.

Parameters

This command has no parameters.

Example

/back

Inaccurate

Since Minecraft has no method for actual teleportation that's widely used, the back command assumes you've teleported when you've either moved a large distance in a single tick, if you changed dimensions or if you died.
This means all sorts of nonsense will happen if you're cruising around your world at mach 10 using the Speed Command.