A “church” is a Christian house of worship. Churches, like any other house of worship, vary largely in size and structure, but all are known as the “house of the Lord” or “God’s house.” That being said, the Christian belief that any structure can become a church has been a growing theological trend in recent years, referencing the early church in the book of Acts, which met in homes, and based on the scripture of Matthew 18:20:
For where two or three are gathered in my name, there am I in the midst of them.
This is largely a Protestant movement, as the Catholic church has rather strict regulations for church structures and for what makes a basilica. A Catholic church is only called a “basilica” with a special designation by the Pope and with the requirement to celebrate particular feast days and services. The term "basilica" comes from the early Roman church, but it was originally used to designate other buildings of public assembly.