The term 'Apple Invader' refers to a group of four different invaders who attacked Europe in the Middle Ages. The first invader was the WORM, a Germanic tribe that invaded the Roman Empire in the 4th century. The second invader was the TATAR, a Mongolian tribe that invaded Eastern Europe in the 13th century. The third invader was ATTILA, the leader of the Huns who invaded Europe in the 5th century. Finally, the fourth invader was the GOTH, a Germanic tribe that invaded the Roman Empire in the 3rd century. Together, these four invaders are known as the Apple Invaders, as they all played a role in the invasion of Europe during the Middle Ages.