When it comes to describing something that is not fixed or constant, there are numerous synonyms that can be used in place of the phrase "is variable". Some good options include words like fluctuating, changing, unpredictable, unstable, and shifting. Each of these alternatives provide slightly different nuances to the same idea, allowing for a greater degree of precision and accuracy in language. For example, describing a situation as "fluctuating" suggests a regular pattern of ups and downs, whereas "unstable" implies that it could collapse or fail at any moment. Ultimately, choosing the right synonym for "is variable" depends on the specific context and desired meaning.