indirect abortion is not what most people mean when they say 'abortion'. However, this is the term used by moral theologians and in some Church documents.
Murder is the direct and voluntary (deliberate) killing of an innocent human being; it is morally direct because the deliberately chosen act is inherently directed at the death of the human person.
indirect abortion occurs when the death of the prenatal is not intended AND is not a direct result of the chosen act. The death of the prenatal is an unintended bad consequence (in the third font of morality). The chosen act is NOT the killing of the prenatal, but the treatment of a disease afflicting the mother.
For example, a woman is in the first trimester of pregnancy and she learns that she has cancer. If she waits to receive the cancer treatment (until the prenatal is viable outside the womb and is delivered), the cancer will kill both the mother and the child. If she receive the cancer treatment, the prenatal will not survive, but the mother may survive. Taking this treatment is moral, even though the prenatal is killed by the treatment, because the intention is to save the life of the mother (good first font), the chosen act has the moral object of saving the life of the mother by giving her a cancer treatment; this treatment is not inherently directed at the death of the prenatal, but at the health of the mother. In the third font, the good consequences (the possibility of saving the mother's life) outweighs the bad consequence of the death of the prenatal because the prenatal's life cannot be saved no matter what is done.
This is referred to as indirect abortion. Although indirect abortion is not intrinsically evil, it is still only moral if all three fonts of morality are good. So if there is a reasonable chance of saving the prenatal's life, by waiting until the prenatal is viable before beginning the cancer treatments, then they must wait. Indirect abortion would not be moral in every circumstance.
Roman Catholic theologian