Conventional: A raised structure on which are performed acts of worship or sacrifice towards a deity.
ACIM: An inner altar, not an outer altar. The place in your mind that contains what you are devoted to, worship, consider sacred. “These altars are not things; they are devotions” (T-5.II.8:7). You have placed the ego’s idols upon your inner altar and worshipped them, yet it is God Who really belongs on your altar and Who really is there. Sometimes spoken of as a single altar that is (only apparently) defiled; sometimes spoken of as two altars, one to the ego and one to God. See T-5.II.8:5-9, T-11.VI.5:1-2.