Elder Scrolls Plugin (.esp)
An Elder Scrolls Plugin (or plugin) is a file type with the extension .esp. Plugins are the main method of creating mods for Morrowind. A plugin contains a hierarchy of records that define the alterations made to the parent master file. Though a plugin can have multiple parent master and plugin files (dependencies). The Construction Set is required to create one, though there are a few third party tools that also have the capability.
Essentially, a plugin file is a database of changes made to its parents.
The top level record of a plugin contains the filename, the author details, description, file version, and parent files.