Can refer to a physical link in a chain or a figurative link in a series.
El eslabono perdido de la cadena era esencial para la seguridad del puente que conectaba las dos orillas.
The missing link of the chain was essential for the safety of the bridge connecting the two shores.
El eslabono que faltaba en la cadena hizo que todo el sistema se desmoronara inesperadamente.
The missing link in the chain caused the entire system to collapse unexpectedly.