Catalyst
Etymology
Drawn from Greek katalysis, "a dissolving" (kata-, "down," plus lyein, "to loosen"). The Swedish chemist Jöns Jacob Berzelius introduced "catalysis" in 1835 for substances that trigger reactions without being used up; "catalyst" is the back-formed name for such a substance. The everyday sense — a trigger that starts something bigger — followed naturally.
noun · /KAT-uh-list/
Something that speeds up a change without being consumed by it, lowering the effort required to get a reaction going.
What it names
The person, event, or condition that makes a long-stalled change suddenly happen — while itself remaining largely unchanged afterward.
Example
One frank comment from a new hire was the catalyst for a conversation the office had avoided for years.
Related idea
A catalyst only lowers the barrier to a change already possible; a threshold is the barrier itself — the level the situation must cross before anything happens.