What is the X in X-mas?
Old English alter, altar, from Latin altare (plural altaria) "high altar, altar for sacrifice to the great gods," perhaps originally meaning "burnt offerings" (cf. Latin adolere "to worship, to offer sacrifice, to honor by burning sacrifices to"), but influenced by Latin altus "high." In Middle English, often auter, from Old French auter. Reintroduced from Latin 1500s. As a symbol of marriage, by 1820.
(Heb. mizbe'ah, from a word meaning "to slay"), any structure of earth (Ex. 20:24) or unwrought stone (20:25) on which sacrifices were offered. Altars were generally erected in conspicuous places (Gen. 22:9; Ezek. 6:3; 2 Kings 23:12; 16:4; 23:8; Acts 14:13). The word is used in Heb. 13:10 for the sacrifice offered upon it--the sacrifice Christ offered. Paul found among the many altars erected in Athens one bearing the inscription, "To the unknown God" (Acts 17:23), or rather "to an [i.e., some] unknown God." The reason for this inscription cannot now be accurately determined. It afforded the apostle the occasion of proclaiming the gospel to the "men of Athens." The first altar we read of is that erected by Noah (Gen. 8:20). Altars were erected by Abraham (Gen. 12:7; 13:4; 22:9), by Isaac (Gen. 26:25), by Jacob (33:20; 35:1, 3), and by Moses (Ex. 17:15, "Jehovah-nissi"). In the tabernacle, and afterwards in the temple, two altars were erected. (1.) The altar of burnt offering (Ex. 30:28), called also the "brasen altar" (Ex. 39:39) and "the table of the Lord" (Mal. 1:7). This altar, as erected in the tabernacle, is described in Ex. 27:1-8. It was a hollow square, 5 cubits in length and in breadth, and 3 cubits in height. It was made of shittim wood, and was overlaid with plates of brass. Its corners were ornamented with "horns" (Ex. 29:12; Lev. 4:18). In Ex. 27:3 the various utensils appertaining to the altar are enumerated. They were made of brass. (Comp. 1 Sam. 2:13, 14; Lev. 16:12; Num. 16:6, 7.) In Solomon's temple the altar was of larger dimensions (2 Chr. 4:1. Comp. 1 Kings 8:22, 64; 9:25), and was made wholly of brass, covering a structure of stone or earth. This altar was renewed by Asa (2 Chr. 15:8). It was removed by Ahaz (2 Kings 16:14), and "cleansed" by Hezekiah, in the latter part of whose reign it was rebuilt. It was finally broken up and carried away by the Babylonians (Jer. 52:17). After the return from captivity it was re-erected (Ezra 3:3, 6) on the same place where it had formerly stood. (Comp. 1 Macc. 4:47.) When Antiochus Epiphanes pillaged Jerusalem the altar of burnt offering was taken away. Again the altar was erected by Herod, and remained in its place till the destruction of Jerusalem by the Romans (70 A.D.). The fire on the altar was not permitted to go out (Lev. 6:9). In the Mosque of Omar, immediately underneath the great dome, which occupies the site of the old temple, there is a rough projection of the natural rock, of about 60 feet in its extreme length, and 50 in its greatest breadth, and in its highest part about 4 feet above the general pavement. This rock seems to have been left intact when Solomon's temple was built. It was in all probability the site of the altar of burnt offering. Underneath this rock is a cave, which may probably have been the granary of Araunah's threshing-floor (1 Chr. 21:22). (2.) The altar of incense (Ex. 30:1-10), called also "the golden altar" (39:38; Num. 4:11), stood in the holy place "before the vail that is by the ark of the testimony." On this altar sweet spices were continually burned with fire taken from the brazen altar. The morning and the evening services were commenced by the high priest offering incense on this altar. The burning of the incense was a type of prayer (Ps. 141:2; Rev. 5:8; 8:3, 4). This altar was a small movable table, made of acacia wood overlaid with gold (Ex. 37:25, 26). It was 1 cubit in length and breadth, and 2 cubits in height. In Solomon's temple the altar was similar in size, but was made of cedar-wood (1 Kings 6:20; 7:48) overlaid with gold. In Ezek. 41:22 it is called "the altar of wood." (Comp. Ex. 30:1-6.) In the temple built after the Exile the altar was restored. Antiochus Epiphanes took it away, but it was afterwards restored by Judas Maccabaeus (1 Macc. 1:23; 4:49). Among the trophies carried away by Titus on the destruction of Jerusalem the altar of incense is not found, nor is any mention made of it in Heb. 9. It was at this altar Zacharias ministered when an angel appeared to him (Luke 1:11). It is the only altar which appears in the heavenly temple (Isa. 6:6; Rev. 8:3,4).