1.action that makes it pointless or impossible for somebody else to do what he or she intended
2.the occupation of public land in order to have the right to buy it later, or the right to buy that is gained in this way
3.an option to purchase property if and when it is put up for sale
4.the strategy of attacking an enemy in order to prevent that enemy from attacking first
5.the doctrine that prohibits a state from enacting laws in an area if the U.S. federal government has passed laws in that area
preemptions