About
A ligature 1) is a font feature that allows the rendering of an icon glyph simply by using its textual name.
A ligature maps a combination of basic characters to represent another character (code point).
The replacement is done by the application (in the example below by the web browser) and provides more readable code.
Example
Icon
Icon names are ligature.
The Material Icons font has the ligature add that points to the character 
Example:
The icon <mark>add</mark> given by its name: <span class="icon">add</span>
- Result: