Which word is similar to interior?

Words similar to interior depend on whether you mean noun meaning inside space or adjective meaning inner. For noun interior meaning inside part of building or object, similar words include inside, inner part, inner space, inner side, core, center, heart, inner room, and indoors. For example, interior of house is similar to inside of house. For interior as design field, similar words include inner decor, inside design, internal design. For adjective interior meaning located inside, similar words include inner, internal, inside, inward, innermost, internalized, and indoor. For example, interior walls is similar to inner walls or internal walls. For figurative meaning of interior as inner thoughts or character, similar words include inner self, inner being, psyche, soul, mind, and internal world. Opposite of interior is exterior. So most common similar words are inside for noun and inner or internal for adjective. For example, you can say inner part instead of interior, internal area instead of interior area. In architecture, interior is often interchangeable with inside and inner space. Choose synonym based on context, but inside, inner, and internal are closest general similar words to interior.