Merriam-Webster Collegiate Dictionary - preemption

 noun  Etymology: Medieval Latin praeemption-, praeemptio previous purchase, from praeemere to buy before, from Latin prae- pre- + emere to buy — more at redeem  Date: 1602  1.  a. the right of purchasing before others; especially one given by the government to the actual settler upon a tract of public land  b. the purchase of something under this right  2. a prior seizure or appropriation ; a taking possession before others