council · 325 · Nicaea
Constantine convened the bishops at Nicaea in 325 — tradition counts 318 fathers — to end the Arian dispute. The council confessed the Son homoousios (of one being) with the Father, condemned and exiled Arius, fixed a common Sunday for Easter, and issued 20 canons confirming the ancient precedence of Rome, Alexandria and Antioch. The first council the whole church counts ecumenical.
The First Council of Nicaea was a council of Christian bishops convened in the Bithynian city of Nicaea by the Roman Emperor Constantine I, also known as the First Ecumenical Council. It met from May until the end of July 325.
The Arian question — is the Son fully God, of the Father's own being? Also the date of Easter and the Melitian schism.
Arius condemned and exiled; the Son confessed homoousios — of one being with the Father — in the creed; Easter fixed on a common Sunday; 20 canons of order confirming the precedence of Rome, Alexandria and Antioch.
Wikipedia · Catholic Encyclopedia — First Council of Nicaea · Fordham sourcebook — documents of Nicaea I · OrthodoxWiki — First Ecumenical Council · The Seven Ecumenical Councils — Nicaea I (NPNF II/14)