The medical definition of an abortion is a pregnancy which ends before fetal viability. Naturally this is at 28 weeks; 24 weeks is now the legal limit.
Until 1992 when the law was changed to make 24 weeks the legal limit for an induced abortion, the accepted limit for a naturally occurring abortion or miscarriage was 28 weeks. Nowadays the 24 week limit is applied to miscarriages as well.
An abortion may be spontaneous or induced.
Spontaneous abortions are usually classified as first or second trimester, with 13 weeks as the division.
Induced abortions are very early abortions and those under 10 weeks, early abortions are those taking place at 10-19 weeks, late abortions are those 20 weeks and over.