Abortion was a procedure in which a fetus was terminated within the mother's womb.
In 2153, the Triannon female Indava asked Doctor Phlox to terminate her pregnancy, as she did not want her child to become a soldier in D'Jamat's war. Phlox did not perform the procedure because he was not finished "studying their physiology." (ENT: "Chosen Realm")
In 2365, Deanna Troi became pregnant from a non-corporeal lifeform. Lieutenant Worf proposed aborting the fetus as it may have been a threat to the crew. Troi refused, and carried the child to term, which lasted just over a day. (TNG: "The Child")