The NetHack encyclopedia is NetHack's built-in database of information about in-game monsters, objects and features. It can be accessed using the what is command: /, and then choosing not to specify an unknown object by cursor. You can then type in the name of anything you like.
Some entries, like that for the mind flayer, are written by the DevTeam as explicit explanations of what the monster does. Others, such as that for lich, are cryptic, in the form of a literary extract. Be warned that these are not always an accurate guide to NetHack's version of the monster/object.
Not all objects have an encyclopedia entry.
The encyclopedia is stored in the data.base file in the dat folder, in text format. On a Windows machine, it can be opened with WordPad (Notepad will not correctly display the line returns, resulting in a jumble of text). Instructions on editing the file can be found at the top. Once you've edited the file, you will need to recompile it. The normal nethack build procedure will do this.