Where is a “mortal sin” defined? In other words, by what authority are sins classified as such?
The Catechism of the Catholic Church defines a mortal sin as follows:
"Mortal sin destroys charity in the heart of man by a grave violation of God's law; it turns man away from God, who is his ultimate end and his beatitude, by preferring an inferior good to him." (C.C.C. # 1855)
"Mortal sin, by attacking the vital principle within us - that is, charity - necessitates a new initiative of God's mercy and a conversion of heart which is normally accomplished within the setting of the Sacrament of Confession." (C.C.C. # 1856)
"Mortal sin is a radical possibility of human freedom, as is love itself. It results in the loss of charity and the private of sanctifying grace, that is, of the state of grace. If it is not redeemed by repentance of God's forgiveness, it causes exclusion to make choices for ever, with no turning back. However, although we can judge that an act is in itself a grave offense, we must entrust judgment of persons to the justice and mercy of God." (C.C.C. # 1861)
"To choose deliberately - that is, both knowing it and willing it - something gravely contrary to the divine law and to the ultimate end of man is to commit a mortal sin. This destroys in us the charity without which eternal beatitude is impossible. Unrepented, it brings eternal death." (C.C.C. # 1874)
In simple terms for a sin to be mortal, the individual must know it is a mortal sin and agree to do it anyway. Does that answer your question?