Inconvertibleness is a term that refers to the quality or state of not being able to be converted or changed. Some possible synonyms for this word include immutability, unalterability, unchangeableness, fixedness, steadfastness, and stability. These terms all convey the idea of something that is permanent or enduring, and cannot be modified or transformed in any way. Other synonyms for inconvertibleness might include permanence, constancy, permanency, durability, and longevity. These words can all be used to describe things that are stable and unchanging, such as the laws of nature, fundamental principles of science, or the enduring qualities of human nature.