Unmodifiable is a term used to describe something that cannot be changed or altered in any way. It is a concept widely used in software programming, where certain values must remain constant throughout a program. However, there are several synonyms for unmodifiable that can be used interchangeably to describe this concept. Immutable, fixed, rigid, inflexible, and unalterable are all terms that describe something that cannot be changed. These terms can be used to describe physical objects as well as ideas and concepts. It is important to choose the right synonym that fits the context of the sentence to ensure clear communication.