Ineffabilis Deus
Ineffabilis Deus (Latin for 'Ineffable God') is an apostolic constitution[1][2] by Pope Pius IX.[3]
It defines the dogma of the Immaculate Conception of the Blessed Virgin Mary.
The document was promulgated on December 8, 1854,[4] the date of the annual Solemnity of the Immaculate Conception, and followed from a positive response to the encyclical Ubi primum.
Mary's immaculate conception is a pronouncement made ex cathedra and is therefore considered by the Catholic Church to be infallible through the extraordinary magisterium.
Context
During the reign of his predecessor, Pope Gregory XVI, the bishops in various countries began to press for a definition as dogma of the teaching of Mary’s immaculate conception.[5]
Pius then appointed a commission to study the matter; in 1849, in the encyclical Ubi primum, he solicited the opinions of the bishops.