Vimscript
Note that the language was re-engineered for Vim 9+. For details on the new language, see Vim9script
Example
With an interactive vim(1) session, try typing:
:echo "Hello, world!"
The leading colon (:) triggers command-line mode, which exposes traditional ex commands and the Vimscript REPL.
Language
See also
Learn Vimscript the Hard Way, the definitive reference for Vimscript