From Longman Dictionary of Contemporary Englishmissing linkˌmissing ˈlink noun [countable] 1 SOLVE/FIND THE ANSWERa piece of information that you need in order to solve a problem Could this be the missing link in the search for a cure for cancer?2 → the missing link
Examples from the Corpus
missing link• Police continue looking for missing links in the Stewart murder case.• Here the missing link is frequently the directly observed contextual detail which is so crucial in anthropological field work.• The modern expression does not include the missing link, spirituality.• David Henshaw of Bristol University says the discovery is the missing link that shows how power lines can cause cancer clusters.• The steam engine was the missing link. 