"Let in" is a phrasal verb that refers to granting access or permission to enter a certain place or area. It is a common expression in everyday conversations and is often used interchangeably with other synonyms. Some of the synonyms of "let in" include permit, allow, admit, grant, authorize, and welcome. Permit is a formal term used to allow or give permission for someone or something to enter, while allow is a more general term used to permit or enable an action or behavior. Admit and grant are more formal synonyms that refer to giving access or permission for something or someone to enter a certain place or location. Authorize and welcome are synonyms that denote an official endorsement or an act of hospitality toward someone or something entering.