A church, church building, or chapel is a building used for Christian worship services and Christian activities. The earliest identified Christian church is a house church founded between 233 AD and 256 AD. The word church also describes a body or assembly of Christian believers, while "the Church" refers to the worldwide Christian religious community.

In traditional Christian architecture, the plan view of a church often forms a Christian cross. The center aisle and seating create the vertical beam, while the bema and altar form the horizontal arms. Towers or domes rise above the heaven-facing roof line to encourage contemplation of the divine. Modern churches employ varied architectural styles, and many buildings originally designed for other purposes have been converted to churches. From the 11th to the 14th centuries, Western Europe experienced a wave of church construction.

Many churches worldwide are of considerable historical, national, cultural, and architectural significance. Several are included in the list of UNESCO World Heritage Sites.

The word church is derived from Old English cirice, 'place of assemblage set aside for Christian worship', from the Common Germanic word kirika. This was probably borrowed via Gothic from Ancient Greek kyriakon doma, 'the Lord's (house)', from kyrios, 'ruler, lord'. Kyrios in turn comes from the Indo-European root *ḱewh₁-, meaning 'to spread out, to swell' (euphemistically: 'to prevail, to be strong').

The word's cognates in many languages reflect its transmission from Greek and Proto-Indo-European roots. In early Germanic languages such as Old High German, the term became kirihha, signaling how Christianization shaped local vocabulary. Early Christian communities used the word to stress a building's dedication to God.

The Greek kyriakon, 'of the Lord', has been used of houses of Christian worship since c. AD 300, especially in the East, although it was less common in this sense than ekklesia or basilike.

The history of church buildings traces the transformation of Christian worship spaces from clandestine house churches in the Roman Empire to monumental basilicas after legalization in 313 with the Edict of Milan, when imperial patronage and civic basilica forms were adapted to liturgical needs. During the 10th to 12th centuries the Romanesque period emphasized thick masonry walls, barrel and groin vaults, and round arches, followed in the 12th to 16th centuries by Gothic architecture, which developed pointed arches, rib vaults, and flying buttresses to achieve greater height and light. From the 15th century Renaissance architecture revived classical orders, symmetry, and proportional systems, and in the 17th to 18th centuries Baroque architecture and Rococo churches used theatrical space, integrated decoration, and urban scenography in response to varied patronage including the Counter-Reformation. After 1517 the Reformation fostered preaching-oriented halls and centralized plans in many Protestant regions, while the Orthodox East sustained and elaborated domed cross-in-square and other centralized schemes. Global expansion carried European models and local adaptations to the Americas, Africa, and Asia, and the 19th century saw widespread historic revivals. In the 20th century new materials, modernist minimalism, and liturgical reforms such as the Second Vatican Council of 1962 to 1965 reshaped altars, seating, and the relationship between clergy and laity.

The earliest archeologically identified Christian church is a house church (domus ecclesiae), the Dura-Europos church, founded between 233 AD and 256 AD.