Deborah is, as always, the diva of doctrine, but allow me to flesh it out a bit.
The term itself comes from Pope Pius IX; it was given in the presence of more than 200 bishops on December 8, 1856, and was declared official Catholic dogma.
It is very important to know that this is not the same as the miracle of the virgin birth but is very closely associated with it. Instead, Immaculate Conception states that she (Mary) "...was preserved exempt from all stain of original sin...", and as such was her child, Jesus, to preserved. The traditionally known essence of original sin was not removed from her soul, but was completely excluded from her soul. The state of original sanctity, innocence, and justice, as opposed to original sin, was conferred upon her.
It is from this that her more common title, the Blessed Virgin, comes from; that she singularly was given a special blessing from God to facilitate a special purpose.