Not susceptible to a process of decay, such as radioactivity. For example, the most common isotope of carbon, carbon 12, is stable. Protons and photons are examples of stable subatomic particles. See more at decay.
Relating to a chemical compound that does not easily decompose or change into other compounds. Water is an example of a stable compound.
Relating to an atom or chemical element that is unlikely to share electrons with another atom or element.
Not likely to change significantly or to deteriorate suddenly, as an individual's medical condition.