A miscarriage is medically considered an abortion which is why the original Hippocratic Oath says that he will not "give a woman a pessary to cause an abortion."
Sometimes you will hear a doctor speak of an "induced abortion" and a "natural abortion".
Actually, I was being a bit pedantic.
The common use of the term, which is what determines its definition, has changed. Nobody outside the medical profession uses the word in its original sense anymore. The dictionaries just haven’t caught up yet.