metastable
adjective
meta·sta·ble ˌme-tə-ˈstā-bəl 
: having or characterized by only a slight margin of stability
a metastable compound
metastability
ˌme-tə-stə-ˈbi-lə-tē
noun metastably
ˌme-tə-ˈstā-b(ə-)lē
adverb 
Example Sentences
Recent Examples on the Web In 2000, astronomers predicted that metastable helium should show up in certain planetary atmospheres, but a search for it—reported in 2003,—on the planet HD 209458b, came up empty. 
Alexandra Witze, Scientific American, 4 May 2018 Guth wondered what would have happened if such a metastable phase transition occurred in the early universe. Adam Davidson, The New Yorker, 23 Mar. 2017
Alexandra Witze, Scientific American, 4 May 2018 Guth wondered what would have happened if such a metastable phase transition occurred in the early universe. Adam Davidson, The New Yorker, 23 Mar. 2017 Word History
Etymology
International Scientific Vocabulary
First Known Use
1897, in the meaning defined above
Medical Definition
metastable
adjective
meta·sta·ble ˌmet-ə-ˈstā-bəl
: having or characterized by only a slight margin of stability
a metastable compound