The six to eight week period in which a woman recovers after giving birth. "Recovers" here means that the internal organs return to their original locations, the vagina and uterus contract and become less tender, and that the menstrual period resumes. In short, while the new mother is no longer pregnant, her body needs time to adjust to this.
The term "puerperium" is derived from the Latin "puerpera": "puer", meaning "child", plus "parere", meaning "to give birth". The adjective pertaining to this period of recovery is "puerpueral". Thus in the postpartum period women may have puerpueral disorders, one of the most common being puerpueral fever.
Physical recovery after childbirth varies in time. For instance, the uterus becomes firm again in five to seven days, but it is not back to its original state until the full six to eight weeks of the puerperium have passed. A large variant is breastfeeding, which influences the return of the period by suppressing ovulation. For those who are not breastfeeding, the period will return six weeks after delivery; for those who are breastfeeding, the time may stretch up to twelve weeks. On the other hand, you have ten to twelve days of various discharge immediately after the birth, so it all evens out.
Sources:
http://www.merck.com/pubs/mmanual/section18/chapter254/254a.htm
http://www.MedTerms.com/script/main/art.asp?li=MNI&ArticleKey=7922