Lua API Documentation
The Lua API of Minetest is extensively documented. This page tells you which documentation exists, and where to find it.
Official documentation
lua_api.txt / HTML
This is the most important document you need during modding.
This is a text file containing the only (!) official reference documentation for the entire Lua API. You can find it in the doc directory of your Minetest installation, or available online using the links below. We strongly recommend you read the file installed on your computer so you read the correct version
Latest (in-development) version available online as HTML or plaintext.
Note this assumes you are already familiar with Lua programming. If you aren't, take a look into the Minetest Modding Book first.
Other text files
In the doc directory of your Minetest installation, you will find a few other text files with related, but less important information and documentation.
Unofficial Documentation
Minetest Modding Book
A very extensive introduction into Minetest modding, great for complete beginners. Written by rubenwardy, a long-time developer for Minetest.