בית הפרס (Bet HaPrass or Beis Haprass) is a term used by the Mishna to define a field that contained human remains that has been plowed – thus creating a בית הפרס where Chazal where concerned that bone fragments where dispersed throughout a specified area thus rendering the entire area forbidden for entry to a kohen.
Chazal enacted that the very earth for an area of 100 amot (apprx. 157.5 feet) from this field is מטמא (renders impurity) just as a dead body is מטמא – it is possible to purify a בית הפרס if the said field is thoroughly checked to ascertain that all human remains have been removed.
Although the earth from the said field is מטמא, the tumah of ohel is inapplicable (Mishna Oholot, ch. 2 mishna 3).
See מסכת אהלות ch. 17 for more information.