Church
From the Compendium Catechism:

147. What does the word Church mean?

The word Church refers to the people whom God calls and gathers together from every part of the earth. They form the assembly of those who through faith and Baptism have become children of God, members of Christ, and temples of the Holy Spirit.

148. Are there other names and images with which the Bible speaks about the Church?

In Sacred Scripture we find many images which bring out various complementary aspects of the mystery of the Church. The Old Testament favors those images that are bound to the people of God. The New Testament offers images that are linked to Christ as the Head of this people which is his Body. Other images are drawn from pastoral life (sheepfold, flock, sheep), from agriculture (field, olive grove, vineyard), from construction (dwelling place, stone, temple), and from family life (spouse, mother, family). 

149. What is the origin and the fulfillment of the Church?

The Church finds her origin and fulfillment in the eternal plan of God. She was prepared for in the Old Covenant with the election of Israel, the sign of the future gathering of all the nations. Founded by the words and actions of Jesus Christ, fulfilled by his redeeming death and Resurrection, the Church has been manifested as the mystery of salvation by the outpouring of the Holy Spirit at Pentecost. She will be perfected in the glory of heaven as the assembly of all the redeemed of the earth.

150. What is the mission of the Church?

The mission of the Church is to proclaim and establish the Kingdom of God begun by Jesus Christ among all peoples. The Church constitutes on earth the seed and beginning of this salvific Kingdom.

151. In what way is the Church a mystery?

The Church is a mystery in as much as in her visible reality there is present and active a divine spiritual reality which can only be seen with the eyes of faith.

152. What does it mean to say that the Church is the universal sacrament of salvation?

This means that she is the sign and instrument both of the reconciliation and communion of all of humanity with God and of the unity of the entire human race.