There are several antonyms for the term "unstable" which can be used to describe something that is firm, solid, or steady. For instance, the word "stable" is an obvious antonym for "unstable". This term implies that something is firmly rooted and not likely to shift or move. The term "steady" is another antonym which can be used to describe something that is predictable and consistent. Other antonyms for "unstable" include words such as "secure", which describes something that is safe and protected, and "fixed", which implies that something is firmly established and not subject to change. Additionally, the term "balanced" is an antonym, which is often used to describe something with equal weight, stability, and proportion.