Modal editing is a text-editing paradigm in which the same keys perform different actions depending on the editor’s current mode. It was popularized by the Unix visual editor vi and carried forward by Vim and Neovim. Rather than dedicating modifier keys to every operation, a modal editor switches the meaning of the entire keyboard between a small number of modes, so plain letter keys can act as a dense command language in one mode and as literal text input in another.
Vim’s own documentation enumerates the core modes. In Normal mode, per the help file, “you can enter all the normal editor commands”; this is the mode the editor starts in and is “also known as command mode.” Insert mode is where “the text you type is inserted into the buffer.” Visual mode “is like Normal mode, but the movement commands extend a highlighted area,” so an operation can be applied to a selected region. Command-line mode lets the user “enter one line of text at the bottom of the window” for Ex commands introduced with ’:’, for the ’/’ and ’?’ search patterns, and for the ’!’ filter command.
The power of the model comes from composition in Normal mode. Commands are typically built from an operator (such as delete, change, or yank) combined with a motion or text object that describes what the operator acts on (a word, a line, a paragraph, the region up to a search match). Because operators and motions combine freely, a relatively small set of keys produces a large vocabulary of edits, and counts can be prefixed to repeat them. This grammar-like structure is what experienced users mean when they describe modal editing as fast and expressive.
The trade-off is a steeper initial learning curve, since users must always be aware of which mode they are in and must deliberately switch into Insert mode to type ordinary text. Modal editing traces back through the vi lineage to line editors like ed and ex, where command and input were already distinguished, and vi made the model interactive on a full screen. Today the approach lives on not only in Vim and Neovim but also in the modal emulation modes offered by many modern editors and IDEs, a sign of how durable the design has proven.