Altar in Eastons Bible Dictionary

(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...

Link: https://bible-history.com/eastons/A/Altar/