ˌmissing ˈlink noun [countable]

1. a 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 an animal which was a stage in the development of humans from apes, whose bones have not yet been found

[TahlilGaran] Dictionary of Contemporary English ▲

missing link
n.
1. Something needed to complete a group; a missing part of a chain of things.
A 1936 penny was the missing link in John's collection of pennies.
The detective hunted for the fact that was the missing link in the case.
2. An unknown extinct animal that was supposed to be a connection between man and lower animals.
The missing link would be half man and half ape.

[TahlilGaran] English Idioms Dictionary ▲