According to About.com, the legal term "per stirpes" means "by representation" in Latin and is used in Last Wills and Testaments and Revocable Living Trusts. The common usage is "I leave my estate to my then living descendants, per stirpes."
About.com explains that upon distributing a person's estate "per stirpes," each branch of the person's family receives an equal share of the estate. If a first-generation heir precedes the decedent in death, the equal share would be split equally among the heir's issue. If the first-generation heir did not have any descendants, his share of the estate would be split among the remaining first-generation heirs equally.