Total conversion is a term used to describe mods that replace vanilla Minecraft's mechanics and progression with their own, greatly changing the way the game is played.
The first major total conversion mod was Better Than Wolves. Although it was not originally created as a total conversion mod, but it later morphed into one. Other total conversion mods include TerraFirmaCraft, GregTech 5 and 6, DivineRPG, and The Mists of RioV.
Modpacks generally do not include total conversion mods, but there are a few modpacks that include them. These modpacks usually use the total conversion mod as the focus of the modpack, and balance other mods around it. Examples include TerraFirmaPunk and GT New Horizons.
The term is also sometimes used to describe modpacks that replace their mods' mechanics and progression radically, such as Agrarian Skies: Hardcore Quest.