Grace is a participation in the life of God. It introduces us into
the intimacy of Trinitarian life: by Baptism the Christian
participates in the grace of Christ, the Head of his Body. As an
"adopted son" he can henceforth call God "Father," in union with the
only Son. He receives the life of the Spirit who breathes charity into
him and who forms the Church. - CCC 1997
From the "PENNY CATECHISM, 139" God's grace is a supernatural gift of God, freely bestowed upon us for our sanctification and salvation. The other numbers 140, 141, and 249 are added to aid in the understanding of how grace is obtained and what it does in the sanctification and salvation of a person.
139 What is Grace?
Grace is a supernatural gift of God, freely bestowed upon us for our sanctification and salvation.
140 How must we obtain God's grace?
We must obtain God's grace chiefly by prayer and the holy Sacraments.
141 What is prayer?
Prayer is the raising up of the mind and heart to God.
249 What is a Sacrament?
A Sacrament is an outward sign of inward grace, ordained by Jesus Christ, by which grace is given to our
souls.
Definition
GRACE In biblical language the condescension or benevolence (Greek
charis) shown by God toward the human race; it is also the unmerited
gift proceeding from this benevolent disposition. Grace, therefore, is
a totally gratuitous gift on which man has absolutely no claim. Where
on occasion the Scriptures speak of grace as pleasing charm or thanks
for favors received, this is a derived and not primary use of the
term.
As the Church has come to explain the meaning of grace, it refers to
something more than the gifts of nature, such as creation or the
blessings of bodily health. Grace is the supernatural gift that God,
of his free benevolence, bestows on rational creatures for their
eternal salvation. The gifts of grace are essentially supernatural.
They surpass the being, powers, and claims of created nature, namely
sanctifying grace, the infused virtues, the gifts of the Holy Spirit,
and actual grace. They are the indispensable means necessary to reach
the beatific vision. In a secondary sense, grace also includes such
blessings as the miraculous gifts of prophecy or healing, or the
preternatural gifts of freedom from concupiscence.
The essence of grace, properly so called, is its gratuity, since no
creature has a right to the beatific vision, and its finality or
purpose is to lead one to eternal life. (Etym. Latin gratia, favor;
a gift freely given.)
Source: Dictionary: GRACE | Catholic Culture
Endnote
From this definition, we then understand the Immaculate Conception of the Virgin Mary and her sinlessness, Arch-Angel Gabriel greeted her, "Hail Full of Grace", as if by her proper name. She was fully sanctified and fully saved.