What does it mean to be immaculately conceived?

Roman Catholic Answer

Immaculately conceived means "conceived without original sin". Adam and Eve were "immaculate" as they were created without original sin, but they were not conceived (they didn't have a mother and father). The only person who was conceived, in the normal manner, but was conceived immaculately, was the Blessed Virgin Mary. The Immaculate Conception refers to the fact that Mary, from the first moment of her conception was protected from the stain of original sin that every other human being has inherited from Adam and Eve. This honor was due to the fact that she would bear the son of God in her womb, and sin can not be anywhere near His Conception.

Please note that the Immaculate Conception refers exclusively to Mary's conception in the womb of her mother, Anne. It has nothing to do with Jesus' conception.