The term has loosely been used to describe a relatively unstable and transient chemical species or (less commonly) a relatively stable but reactive species. It must therefore not be used without explanation of the intended meaning.
See also: inert, persistent, reactive, unreactive
Source:
PAC, 1994, 66, 1077. 'Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)' on page 1134 (https://doi.org/10.1351/pac199466051077)