An advance in agricultural technology that increased farm production in the middle ages -- the coulter loosens the earth, making it easier to turn the furrow with the plow proper.
Thanks Wikimedia Commons
The original coulter was the blade (#4 in the diagram) -- discs started to be used around 1900. From Middle English colter or culter, from Old English culter and Old French coltre, both from Latin culter, plowshare/knife. Spencer mentions in The Faerie Queene:
"a furrow ... which my coulter hath not cleft"