an instrument resting on three of its iron prongs , while a fourth projected upward , thrown on the ground to impede an enemy's cavalry, a caltrop.

http://lila-erc.eu/data/lexicalResources/LewisShort/id/LexicalSense/n48997.0 an entity of type: LexicalSense

an instrument resting on three of its iron prongs , while a fourth projected upward , thrown on the ground to impede an enemy's cavalry, a caltrop. 
an instrument resting on three of its iron prongs , while a fourth projected upward , thrown on the ground to impede an enemy's cavalry, a caltrop. 

data from the linked data cloud