altar / alter are similar-sounding terms with different meanings (referred to as homophones). To better understand the differences, see below for definitions, pronunciation guides, and example sentences using each term. 👇
altar: (noun) the table in Christian churches where communion is given. (noun) a raised structure on which gifts or sacrifices to a god are made.
alter: (verb) make an alteration to. (verb) remove the ovaries of.
altar: AO·l·t·er
alter: AO·l·t·er
(Examples are Wikipedia snippets under the CC ShareAlike 3.0 license. Definitions drawn from WordNet.)