Altar is a structure before which people worship God and make offerings. Lehi “built an altar of stones” after reaching a valley by a river, then “made an offering unto the Lord” and gave thanks unto God (1 Nephi 2:7).
After Alma established the church at Sidom, the people assembled at their sanctuaries “to worship God before the altar,” watching and praying continually for deliverance from Satan, death, and destruction (Alma 15:17).