In the OT, a place of sacrifice near which animals were slaughtered and on which oblations of corn, wine, and incense were burnt and offered, in the open air. The ‘high places’ (cf. e.g. 2 Kgs. 23: 5) were shrines with a natural kind of altar. Solomon's Temple at Jerusalem had an altar in the courtyard overlaid with bronze (1 Kgs. 8: 64) and Ahaz commissioned a larger altar with horns at the four corners. As places for sacrifice, altars were imbued with a sense of holiness where people would approach God and in some cases also seek refuge (1 Kgs. 2: 28). Altars might be of stone, either shaped or natural (Judg. 6: 19–23), or of earth (Exod. 20: 24), or of metal, as was the altar of incense (Exod. 30: 3). After the Jerusalem Temple was established as the national centre for worship, locally installed altars were officially no longer allowed.
In the NT there are eight references to an altar in the Temple or in the New Jerusalem according to the vision in Rev. (e.g. 8: 5). The altar in the existing Temple at Jerusalem is referred to in Matt. 5: 23–4, for in Herod's Temple there was the new altar which the Maccabees had restored (1 Macc. 4: 41 ff.) after the ‘desolating sacrilege’ of Antiochus Epiphanes had polluted the altar in 167 BCE. The reference in 1 Cor. 10: 21 to ‘the Lord's table’ is probably more a reference to the Eucharist than to any structure used for its celebration, and in Heb. 13: 10 the writer claims that Christians have an altar ‘from which those who officiate in the tent (or tabernacle) have no right to eat’, which would appear to be a reference to the Eucharist contrasted with the sacrifices of Judaism.
In the writings of the early Fathers of the Church, structures on which the Eucharist was celebrated are sometimes called a ‘table’ and sometimes an ‘altar’. They were made of wood, though stone altars were introduced when the Eucharist was celebrated at the tomb of a martyr in the Roman catacombs in the 4th cent. CE.