The medical term for miscarriage is still "spontaneous abortion." Medically, the term "abortion" simply means the termination of a pregnancy when the baby is expelled (or removed) from the uterus, usually following the baby's death. "Spontaneous" means that it just happened, without any apparent cause. A "missed abortion" is when the baby has died but the body for whatever reason did not expel it. In both cases, the baby died a natural death. Sometimes a procedure has to be done to remove the baby, to prevent harm to the mother, but the procedure is not what most of us consider an "abortion" because it is not the killing of a live baby, it's the removal of one that has died.
What most of us call an abortion is, medically, an "elective abortion."
In the case of miscarriages, those terms are used up until about 22 weeks.