Adjoiner, a term used to describe something that is adjacent or next to something else, has several synonyms. One synonym is neighbor, which specifically refers to someone or something that is close to another. Another synonym is adjoining, which means located next to something else. Additionally, the word abutting can be used as a synonym for adjoiner, suggesting something touches or is in direct contact with another. Finally, the term contiguous is a synonym for adjoiner, describing something that is in uninterrupted or continuous contact with something else. These synonyms can be used interchangeably depending on the context of the sentence.