Filioque clause
"Filioque" is two Latin words run together. "Filio" is the ablative case of "filius," which means "son." "Que" tacked onto the end of a word is one of the ways Latin has for saying "and." So the literal translation of the word is "and the Son."
The phrase appears in the version of the Nicene Creed used in the Roman Catholic Church and many other western Christian denominations. The full Latin sentence in which it appears is:
"Et in SpÃritum Sanctum, Dóminum et vivificántem: Qui ex Patre Filióque procédit."
"... and in the Holy Spirit, Lord and Giver of Life, Who proceeds from the Father and the Son."
This phrase does not appear in the original version of the Nicene Creed, which was composed in Greek by the Council of Nicea in 325 AD (Nicea is in Greek territory, near Constantinople) and revised by the Council of Constantinople (381 AD; also a Greek council held near Constantinople).
Catechism of the Catholic Church
246 The Latin tradition of the Creed confesses that the Spirit "proceeds from the Father and the Son (filioque)". The Council of Florence in 1438 explains: "The Holy Spirit is eternally from Father and Son; He has his nature and subsistence at once (simul) from the Father and the Son. He proceeds eternally from both as from one principle and through one spiration. . . . And, since the Father has through generation given to the only-begotten Son everything that belongs to the Father, except being Father, the Son has also eternally from the Father, from whom he is eternally born, that the Holy Spirit proceeds from the Son."
247 The affirmation of the filioque does not appear in the Creed confessed in 381 at Constantinople. But Pope St. Leo I, following an ancient Latin and Alexandrian tradition, had already confessed it dogmatically in 447, even before Rome, in 451 at the Council of Chalcedon, came to recognize and receive the Symbol of 381. The use of this formula in the Creed was gradually admitted into the Latin liturgy (between the eighth and eleventh centuries). The introduction of the filioque into the Niceno-Constantinopolitan Creed by the Latin liturgy constitutes moreover, even today, a point of disagreement with the Orthodox Churches.
248 At the outset the Eastern tradition expresses the Father's character as first origin of the Spirit. By confessing the Spirit as he "who proceeds from the Father", it affirms that he comes from the Father through the Son. The Western tradition expresses first the consubstantial communion between Father and Son, by saying that the Spirit proceeds from the Father and the Son (filioque). It says this, "legitimately and with good reason", for the eternal order of the divine persons in their consubstantial communion implies that the Father, as "the principle without principle", is the first origin of the Spirit, but also that as Father of the only Son, he is, with the Son, the single principle from which the Holy Spirit proceeds. This legitimate complementarity, provided it does not become rigid, does not affect the identity of faith in the reality of the same mystery confessed.