Ivy is a generic completion mechanism for Emacs.

From the Ivy Manual:

Ivy is an interactive interface for completion in Emacs. Emacs uses completion mechanism in a variety of contexts: code, menus, commands, variables, functions, etc. Completion entails listing, sorting, filtering, previewing, and applying actions on selected items. When active, ivy-mode completes the selection process by narrowing available choices while previewing in the minibuffer. Selecting the final candidate is either through simple keyboard character inputs or through powerful regular expressions.