A “holy one” who leads a life in union with God through the grace of Christ and receives the reward of eternal life in heaven.
Saints rarely work alone. They work with one another in the Communion of Saints.
A saint may be a man, such as St. John of the Cross, a woman, such as St. Teresa of Avila, or an angel, such as St. Michael the Archangel. What makes them saints is that they are living in heaven. Most of the saints in heaven are ordinary men, women who lived in union with God. For many of the men and women, the only earthly records of their lives on earth are found in dusty church archives and on gravestones. Most of the holy angels in heaven are not known by their individual names even to the Church Militant.
Some Catholic prayers use the phrase, angels and saints, using saint to refer only to human souls in heaven. In the Confiteor we pray, “I confess to almighty God, and to you, my brothers and sisters, that I have sinned through my own fault, in my thoughts and in my words, in what I have done, and in what I have failed to do; and I ask blessed Mary, ever virgin, all the angels and saints, and you, my brothers and sisters, to pray for me to the Lord, our God.” It is accepted usage because we understand what it means, but, strictly speaking, angels, including St. Michael, St. Gabriel, and St. Raphael, are saints.
The Church holds a few saints up for special veneration. The Catechism of the Catholic Church, § 828, says: “By canonizing some of the faithful, i.e., by solemnly proclaiming that they practiced heroic virtue and lived in fidelity to God’s grace, the Church recognizes the power of the Spirit of holiness within her and sustains the hope of believers by proposing the saints to them as models and intercessors. The saints have always been the source and origin of renewal in the most difficult moments in the Church’s history. Indeed, holiness is the hidden source and infallible measure of her apostolic activity and missionary zeal.”
This needs to be said above all else. The Church exists to make us saints. She does not exist to make us feel good about ourselves, or to provide a place for social gathering. She exists to give us food for the journey, the sacraments that will give us the sanctifying grace that open the way to heaven for us. She exists also to teach us how to practice the virtues that prepare us for heaven and to avoid the sins, particularly the capital sins, that block our path to heaven. Overall, she exists to make us saints, to prepare us to enter eternal life in heaven.
That is why the Church is often described as the communion of saints. See the Catechism of the Catholic Church, § 946-962. We walk together on our pilgrim journey toward eternal life in heaven.