By and large, the general public misunderstands the doctrine of the Immaculate Conception, and more often than not, uses the term "Immaculate Conception" incorrectly. This is understandable among non-Catholics who have no background in Catholic theology, but surprisingly enough, many Catholics are just as unaware of the true meaning of the term "Immaculate Conception."
The true meaning of the term "Immaculate Conception" is to be conceived without the stain of original sin. It does not mean to be virginally conceived. This is the chief misunderstanding. Many use the term "Immaculate Conception" when they really mean "Miraculous Conception." All Christians believe, or at least they should, in the doctrine of the Miraculous Conception, which is the virginal conception of Jesus. Jesus was conceived by a virgin without any participation in any kind of sexual activity whatsoever; truly a "miraculous" event.
Over and over again, I've heard "Immaculate Conception" when "Miraculous Conception" was intended. I don't think I've ever heard the term "Miraculous Conception" used in secular movies or television; however, I've heard the term "Immaculate Conception" misused many times. Even Protestants that don't accept the doctrine of Original Sin (as apposed to those that do) usually refer to the conception of Jesus as the "Immaculate Conception."
Some Protestant who do know that the term "Immaculate Conception" means to be conceived without original sin, but do not accept the doctrine of Original Sin, will argue against what they mistakenly believe is the Catholic doctrine of the Immaculate Conception. They will say with fervour determination that Jesus is not the Immaculate Conception because original sin doesn't exist, and therefore there is no need for His conception to be immaculate. This can be somewhat comical to Catholics that know better. These Catholics will agree that Jesus is not the Immaculate Conception; He is the Miraculous Conception. Mary is the Immaculate Conception.
In arguing against the doctrine of Original Sin, these same Protestants will usually point out a flaw in what they mistake for Catholic doctrine. If Mary was conceived immaculately so that Jesus could be conceived immaculately, then Mary's parents would also have to have been conceived immaculately, and their parents, and their parents... Mary was not conceived immaculately so that Jesus could be immaculately conceived. Jesus is God. There is no possible way for Him to be anything but immaculate. If God wanted to, He could have been conceived by Mary even if she wasn't immaculately conceived. Although, I believe that if this were the case, Mary would have to have been immaculate for the nine months that God dwelt in her womb. If she were not, I believe she would have died.
My five minutes is almost up, so I can't even begin to defend the Catholic doctrine of the Immaculate Conception. I do, however, want to make one thing clear, and that is the correct terminology. The conception of Jesus is not the "Immaculate Conception." The conception of Jesus is the "Miraculous Conception." The term "Immaculate Conception" can only be correctly used in reference to the Catholic belief that Mary was conceived without the stain of original sin. Jesus is the Miraculous Conception. Mary is the Immaculate Conception.