Form Zero was a principle followed by many Jedi, not exactly a form of lightsaber combat despite its name. The principle of Form Zero was that a Jedi's most important martial skill was to know to only draw one's lightsaber when absolutely necessary. Jedi Masters who propagated this belief called it "Form Zero" to highlight it as the basis for all other forms of lightsaber combat, building off the central idea that Jedi were keepers of peace and not warriors.
To practitioners of Form Zero, the idea of Jedi warriors learning additional combat expertise, thus gaining temptation to rush towards lightsaber combat over a peaceful resolution, was considered poor technique at best and moral weakness at worst.
- Knights of Fate (First mentioned)