- any of several plants having spiny heads or fruit, as those of the genera Tribulus and Kallstroemia, or the star thistle, Centaurea calcitrapa.
- an iron ball with four projecting spikes so disposed that when the ball is on the ground one of them always points upward: used to obstruct the passage of cavalry, armored vehicles, etc.
- any tropical or subtropical plant of the zygophyllaceous genera Tribulus and Kallstroemia that have spiny burs or bracts
- water caltrop another name for water chestnut (def. 1)
- another name for the star thistle
- military a four-spiked iron ball or four joined spikes laid upon the ground as a device to lame cavalry horses, puncture tyres, etc