I suppose there's 3 logical places that you could pin a definition for political/religious reasons that also have some form of pseudo scientific basis.
1 Sometime in the first week or so when a fertilized egg has been implanted and can feasibly go on to form a baby. You can make a claim for before implantation if you like but in either case, it's within the first few days after The Gross Immoral Act.
2. At the point where medical science can keep a faetus alive outside the womb - this will move nearer to the point of conception as science improves.
3. At the moment of birth.
I don't see how it matters to Catholics though as they regard wearing a condom to be a mortal sin which carries the same penalty as murder in their eyes. For them murder starts even before conception, so wtf do they care when science calls it?
Life, don't talk to me about life - Marvin the Paranoid Android