Loss of ≥1000mL blood in first 24hrs and up to 12wks postpartum with either vaginal delivery or c-section, or bleeding associated with signs/symptoms of hypovolemia within 24 hours of birth 
However, ≥500mL of blood loss after vaginal delivery is abnormal and warrants further investigation
Usually within 24 hours of delivery
If occurs more than 24 hours after delivery, consider: retained POC, coagulopathy, etc
Other pertinent definitions:
Any blood loss to compromising hemodynamic stability
However, ≥500mL of blood loss after vaginal delivery is abnormal and warrants further investigation
Any blood loss to compromising hemodynamic stability
Postpartum hemorrhage
Amniotic fluid embolus
Postpartum endometritis
Chorioamnionitis
Preeclampsia
Acute fatty liver of pregnancy
Septic abortion
Eclampsia