More than a dictionary definition
Abortion is the premature expulsion of the foetus from the womb.
By this definition, miscarriages count as abortions, which explains some unusual statistics like "Half of all pregnancies end in abortion". When talking about the ethics of abortion, it is generally accepted that 'abortion' refers to 'procured abortions', where steps are taken to prematurely end a pregnancy.
Procured abortion can still mean many things, however. It may mean simply taking a pill in your lunch-break which will cause a miscarriage to occur. At later stages, the foetus may need to be cut up inside the womb, or even partially delivered. For Catholics, many contraceptives (such as 'the pill') are concerned to be "abortifacient" (they cause an abortion) by not allowing the embryo to attach to the wall of the womb.
There are disagreements about when a pregancy starts, what counts as a 'foetus' etc. There are also differences of opinion about, for example, how contraceptives work. For more information about abortion, have a look at the Ontario Consultants on Religious Tolerance.
