"Per stirpes" is a Latin term that actually means by branch or by roots. Legally speaking, per stirpes means to divide an estate equally among one's children. This term is found most often, or almost exclusively, in wills and trusts.
When a beneficiary of the testator's will dies before the testator does, the beneficiary's portion of the estate is generally divided per stirpes; that is, it is divided equally among the beneficiary's children. For example, suppose John has two children, Adele and Bruce, and Adele has two children, Charles and Debra. If Adele dies before John, Bruce still receives one half of the estate, and Charles and Debra each receive one quarter. In other words, Charles and Debra share the half that would have been Adele's if she had survived John.