Note only a few are from the Catechism --they are from a wide array of Church documents --to show that this is simply how these terms have been used all over the place.
Mortal Sin = Grave sin= Serious sin
From the Compendium issued by Pope Benedict XVI
"304. Which sins must be confessed?
1456
All grave sins not yet confessed, which a careful examination of conscience brings to mind, must be brought to the sacrament of Penance. The confession of serious sins is the only ordinary way to obtain forgiveness."
(note in the last example those exact terms are used to denote the same thing (mortal sin) -in the very same paragraph)
And in a speech from Pope John Paul II
“The sacrament of Penance is meant to take away personal sins committed after Baptism: first of all
mortal sins, then venial. If the penitent has committed more than one mortal sin, they can only be remitted all at once. In fact, the remission of
serious sin consists in the infusion of the sanctifying grace which has been lost, and grace is incompatible with any and every serious sin. Venial sins are to be regarded differently…”
- What is required to receive Holy Communion?
1385-1389
1415
To receive Holy Communion one must be fully incorporated into the Catholic Church and be in the state of grace, that is, not conscious of being in** mortal sin**. Anyone who is conscious of having committed a
grave sin must first receive the sacrament of Reconciliation before going to Communion.
From:
The Compendium of the Catechism issued by Pope Benedict XVI
CCC1385: “Anyone conscious of a
grave sin must receive the sacrament of Reconciliation before coming to communion.”
“However, it must be remembered that the church, guided by faith in this great sacrament, teaches that no Christian who is conscious of** grave sin** can receive the eucharist before having obtained God’s forgiveness.”
Pope John Paul II Reconciliatio et Paenitentia 17
In MISERICORDIA DEI he also noted:
“Individual and integral confession and absolution are the sole ordinary means by which the faithful, conscious of
grave sin, are reconciled with God and the Church; only physical or moral impossibility excuses from such confession, in which case reconciliation can be obtained in other ways”
and
Since “the faithful are obliged to confess, according to kind and number, **all grave sins **committed after Baptism of which they are conscious after careful examination and which have not yet been directly remitted by the Church’s power of the keys
and in ECCLESIA DE EUCHARISTIA
“If a Christian’s conscience is burdened by
serious sin, then the path of penance through the sacrament of Reconciliation becomes necessary for full participation in the Eucharistic Sacrifice.”
And
Pope Benedict XVI in his Catecheses with Children said:
"Only in that case, when you are in a state of
“mortal” sin,
in other words, grave (sin), is it necessary to go to confession before Communion. This is my first point. "
vatican.va/holy_father/benedict_xvi/speeches/2005/october/documents/hf_ben_xvi_spe_20051015_meeting-children_en.html
“The faithful, in their turn, must seek to receive and to venerate the Most Holy Sacrament with piety and devotion, eager to welcome the Lord Jesus with faith, and having recourse, whenever necessary, to the sacrament of reconciliation so as to purify the soul from every
grave sin.”
vatican.va/holy_father/benedict_xvi/homilies/2007/documents/hf_ben-xvi_hom_20070511_canonization-brazil_en.html