A caltrop is an area denial weapon made up of two or more sharp nails or spines arranged in such a manner that one of them always points upward from a stable base. Historically, caltrops were part of defences that served to slow the advance of troops, especially horses, chariots, and war elephants, and were particularly effective against the soft feet of camels. In modern times, caltrops are effective when used against wheeled vehicles with pneumatic tires.

An area denial weapon is a defensive device used to prevent an adversary from occupying or traversing an area of land, sea or air. The specific method may not be totally effective in preventing passage, but is sufficient to severely restrict, slow down, or endanger the opponent. Some area denial weapons pose risks to civilians entering the area even long after combat has ended, and consequently are often controversial. An area denial weapon can be part of an anti-access/area denial (A2/AD) strategy.