As we know it, "church" means both the building in which we celebrate Mass and the community of the followers of Jesus. Both of these meanings are valid as we can see from the sources of the word.
In English, the word "church" is derived via the Anglo-Saxon circe which comes from the Greek kurios or lord. Like the German kirche or kirika, it comes from the Greek kuriakos which means "thing belonging to the Lord." It is shortened from kuriakon doma meaning the Lord's house, thus the building we know as church.
The words for church in the romance languages such as the Italian chiesa and the French eglise come from the Latin ecclesia which is a transliteration of the Greek verb ekkalio meaning "to summon" or "to call out." Therefore, it is a calling together, a convocation, an assembly.
This word designated an assembly of the people as a political force or of the magistrates or politicians (Acts 19:29-40). This secular meaning influences Paul's use in relation to the gathering or assembly of the Christian community when he scolds the Corinthians for their lack of charity in their gatherings for the Lord's Supper (1 Corinthians 11:18).
In Hebrew, the word for a holy assembly was qahal Yahweh (assembly of God or called together by God). The choice of this word comes from its root kaleo which means call together or convoke indicating that Israel, the people of God was an assembly or gathering called together at God's initiative.
During those special assemblies, God spoke to the people through anointed leaders such as Moses or through holy writings. Qahal is sometimes translated by synagogue, but synagogue is usually the translation for the priestly edah. Both of these terms refer to the cultic assemblies of the people of Israel.
The Septuagint is a translation of the Bible into Greek for Jews who no longer understood Hebrew. Used also by the early Christians, it always translates the Hebrew qahal into the Greek ekklesia (Latin ecclesia).
In the Gospels, it is used twice in Matthew, once referring to the universal Church when Peter is given the leadership (16:18) and a second time to the local Church and its correction of failings in the community (18:17). This word was chosen as it continued the Old Testament idea of a holy convocation of God's people.
After the break between Christians and Jews, edah or synagogue became the term for Sabbath meeting of Jews for prayer and study, as well as the building in which this took place.
Christians took over qahal/ecclesia or church for their assemblies and later their worship places. Early Christians met for the Lord's Supper in homes, that is in house-churches, and so the New Testament never uses this word for a worship place.
The connection between church as a building and as a community goes deeper than one would think. The building is a symbol of the community which is the body of Christ. So, churches were built in the form of a cross and in such a way as to prepare the community to enter more fully into the Eucharistic celebration, the summit of Christian life.
The narthex or porch is the place where one is given the opportunity to pause and prepare to enter not just any building but a very special one. Crossing oneself with holy water at the door of the church proper is a reminder of one's entrance into the life of Christ in Baptism and a re-cleansing similar to a ritual bath.
Before being seated, one reverences the altar, the sacred focus of the liturgy, by bowing or genuflecting. Then, kneeling for a few moments allows one to reflect on the motivation for being there: fear of hell, social acceptance or the only real reason which is to worship God as a community of believers and be strengthened to go out and better serve Christ in the world.
Henri Nouwen tells us that churches are like frames around empty spaces set apart for and witnessing to God who is the quiet still centre of human life. They are tranquil spaces, inviting us in to rest and be silent, listening attentively to God and becoming more aware of the need to create space for God in our daily lives without which our lives as human beings lose their sense of meaning.
For a rich explanation of what Church means, one can read Paul's Letters.
Among other things, he described the Church as God's plantation which depends upon God for its growth (1 Corinthians 3:6-9); God's building whose foundation is Christ (1 Corinthians 3:9-15); God's sanctuary (1 Corinthians 3:16); a new creation (Galatians 6:15); the body of Christ (1 Corinthians 10:16-17, Romans 12:4-8); Christ as the head of that body (Colossians 1:18; Ephesians 5:22-24); the spouse of Christ (Ephesians 5:22-23).
It is quite evident, then, that "church" is profound in meaning, whether we look at it from the point of view of the building or of the community which gathers there to celebrate the sacred mysteries in thanksgiving, praise and adoration, being thereby strengthened to go out and preach the Good News by their lives.