This is the 8887th most frequent Japanese word.
止まり
"Stop" or "halt."
In this sentence, "止まり" is used as a verb meaning "to stop," indicating the cessation of movement of the train at the station.
電車はこの駅に止まりました。
The train stopped at this station.
Here, "止まり" is used as a noun, referring to "stopping," specifically concerning the function of the heart.
心臓の止まりが不安になります。
I’m worried about the stopping of the heart.
In this case, "止まり" represents the action of halting or ceasing, as applied to the functioning of the clock.