Major modes govern how certain commands behave. They adapt a few editor commands so that their use is more appropriate to the text being edited. For example, the concept of indentation is radically different in Lisp mode and Fundamental mode. Each buffer is associated with one major mode.

a customization of Emacs for editing a certain type of text, such as code for a specific programming language

The major modes are a mutually exclusive set of options each of which configures Emacs for editing a certain sort of text. Ideally, each programming language has its own major mode. See section 1/4ç¥â¡1/4¥É.

One of two colorings applied to a key, characterized by the major scale and the resulting predominance of major triads. Generally sounds bright and stable.

a key whose harmony is based on the major scale