Sacred describes something that is dedicated or set apart for the service or worship of a deity; is considered worthy of spiritual respect or devotion; or inspires awe or reverence among believers. The property is often ascribed to objects (a "sacred artifact" that is venerated and blessed), or places ("sacred ground").

French sociologist Émile Durkheim considered the dichotomy between the sacred and the profane to be the central characteristic of religion: "religion is a unified system of beliefs and practices relative to sacred things, that is to say, things set apart and forbidden." In Durkheim's theory, the sacred represents the interests of the group, especially unity, which are embodied in sacred group symbols, or using team work to help get out of trouble. The profane, on the other hand, involve mundane individual concerns.

The word sacred descends from the Latin sacer, referring to that which is 'consecrated, dedicated' or 'purified' to the gods or anything in their power, as well as to sacerdotes. Latin sacer is itself from Proto-Indo-European *seh₂k- "sacred, ceremony, ritual".

Although the English language terms sacred and holy are similar in meaning, and they are sometimes used interchangeably, they carry subtle differences. In the Christian tradition, holiness is generally used in relation to people and relationships, whereas sacredness is used in relation to objects, places, or happenings. For example, a saint may be considered as holy but not necessarily sacred. Nonetheless, some things can be both holy and sacred, such as the Holy Bible.

Although sacred and holy denote something or someone set apart to the worship of God and therefore, worthy of respect and sometimes veneration, holy (the stronger word) implies an inherent or essential character. Holiness originates in God and is communicated to things, places, times, and persons engaged in His Service. Thus, Thomas Aquinas defines holiness as that virtue by which a man's mind applies itself and all its acts to God; he ranks it among the infused moral virtues, and identifies it with the virtue of religion. However, whereas religion is the virtue whereby one offers God due service in the things which pertain to the Divine service, holiness is the virtue by which one makes all one's acts subservient to God. Thus, holiness or sanctity is the outcome of sanctification, that Divine act by which God freely justifies a person and by which He has claimed them for His own.

The English word holy dates back to the Proto-Germanic word hailagaz from around 500 BC, an adjective derived from hailaz ('whole'), which was used to mean 'uninjured, sound, healthy, entire, complete'. In non-specialist contexts, the term holy refers to someone or something that is associated with a divine power, such as water used for baptism.^{[citation needed]}

The concept of things being made or associated with the sacred is widespread among religions, making people, places, and objects revered, set apart for special use or purpose, or transferred to the sacred sphere. Words for this include hallow, sanctify, and consecrate, which can be contrasted with desecration and deconsecration. These terms are used in various ways by different groups.

Sanctification and consecration come from the Latin Sanctus (to set apart for special use or purpose, make holy or sacred) and consecrat (dedicated, devoted, and sacred).