In 3rd edition, a Half Caster is a class that has a caster level of half their class level and only got spells at 4th level. They typically got up to 4th level spells, but had some unique spells or spells gained far earlier than full casters. Paladin, Ranger, and Hexblade were the main half casters. In Pathfinder half casters were upgraded to have a caster level of class level minus 3 instead (and Paladin now uses charisma to cast spells, making them less a victim of Multiple Ability Dependency). Bloodrager follows the format of only getting spells at 4th level, but actually has full caster level progression.
In 5th edition, the above progression is more akin to a Third Caster (confusing, we know) and a Half Caster is instead any class capable of spellcasting with the following traits:
- They start gaining access to spells at 2nd level.
- They can reach up to 5th level spells.
- When multiclassing, it takes 2 levels in this class to gain 1 caster level.
Half Casters in D&D 5e currently consist of the Paladin and Ranger. Artificer is in a unique position of only meeting the second and third traits, placing it in a new tier commonly referred to as a Two-thirds Caster.