altar: the built structure on which an offering is burned and presented to God or to a god
The word is a place word, built straight from the verb meaning "to slaughter for sacrifice," so its plain force is "the place of slaughtering." In practice, though, an altar was seldom where the animal died. The animal was killed beside it; what happened on the altar itself was the throwing of blood against its sides and the burning of the portions until the smoke went up. The name says "slaughter place" while the thing it names works as an offering place: the fixed spot where something is handed over and consumed.
That gap shows up plainly in how the Bible uses the word, because a good many altars never receive an animal at all:
- the small gold altar inside the tent takes incense only, and sacrifice on it is forbidden
- the great altar the eastern tribes raised beside the Jordan was built expressly not for offering but as a witness
- several altars are built and given a name, with no offering described on them
Whatever it is made of, the word stays the same: piled earth, uncut field stones, acacia wood plated with bronze, or a small stand plated with gold. It also stays the same whoever the altar is for. The LORD's altar and a Baal altar are both called this, so the word by itself never tells you whether an altar was approved. What separates them in the Bible is who is worshiped there and whether the place was authorized, never the vocabulary.
It is not the same as a hill shrine (בָּמָה), which is a whole worship site that could hold an altar along with other installations, nor a standing stone (מַצֵּבָה), which marks a place without ever receiving an offering. Ezekiel and Malachi do bring the altar close to the word for a table (שֻׁלְחָן), and that is where the picture of food set before God attaches to it.
All of this makes the altar a kind of gauge of Israel's loyalty. It is where atonement is made and where a fugitive grabs the horns for refuge; it is what the prophets count when they complain that altars have multiplied like the streets of Jerusalem; and it is what Malachi calls the LORD's table, honored or despised by what the priests bring to it.
Nuances
Disputed
Whether the core sense of the word is the place where the victim is slaughtered or the place where offerings are burned and presented.
- contestedThe noun is formed from the verb "to slaughter for sacrifice" and names, literally, the place of slaughtering; the Arabic and wider Semitic parallels all name a slaughtering place.; BDB, s.v. מִזְבֵּחַ; Gesenius, Hebrew and Chaldee Lexicon to the Old Testament Scriptures, s.v. מִזְבֵּחַ
- contestedIsraelite practice killed the animal beside the altar rather than on it, and the defining rite performed on the altar is the burning, so the word functionally names the place where offerings are presented and turned into smoke.; Christian A. Eberhart, Studien zur Bedeutung der Opfer im Alten Testament (WMANT 94); Dohmen, "מִזְבֵּחַ mizbeaḥ," in Theological Dictionary of the Old Testament, vol. 8