An alteration is a change that makes someone or something a little different, or the act of making that change. Plans, rules, and buildings all have alterations. It sounds a step more formal than change. In clothes shops, alterations is almost a set meaning: shortening trousers, taking in a jacket. Those “small” changes can still matter a lot to the person wearing them. The word can cover a process as well as the result. It has nothing to do with an altar in a church — they only sound alike.