key piece
Synonyms
Examples
A good liar knows that the most efficient lie is always a truth that has had a key piece removed from it.
Throughout history there have been many other examples, similar to that of Haeckel, Huxley and the cell, where a key piece of a particular scientific puzzle was beyond the understanding of the age.
I'm the same as a key piece of machinery in a steel mill, which is needed to make steel and to make money for the mill owners. If I break down, it's their problem. My problem is to build some kind of decent life for myself in this crazy unreal world in which I make my living.