The Terror Meter is the sound effect heard in the Amnesia when an enemy is giving Daniel/Justine or Oswald chase. Its name comes from the sound file labeled "ui_terror_meter" in the first game's program files.
In Amnesia: The Dark Descent, the Terror Meter is an extremely loud, high-pitched and somewhat mechanical screeching or whirring noise, like an alarm or a siren. It is the easiest way to determine whether or not a monster is chasing, as it will start once the Gatherer notices Daniel and only stop when losing track. The sound is always accompanied by the appropriate Gatherer attack music. The Terror Meter plays for both Grunts and Brutes. The same will also play when Suitors are pursuing Justine in the expansion.
The Terror Meter is also present in Amnesia: A Machine for Pigs. There are two versions of the sound, one is when Oswald is given chase by a Wretch or an Engineer while the other one is when Oswald is being chased by a Tesla. The first version consists of a loud mechanical screeching or a distorted steaming, while the second one consists of machinery sounds, screeching, and distorted steaming, but with a Tesla-themed style. The sound is also accompanied by the Manpig attack music as well. It is noted that the Terror Meter in this game is considerably quieter than the one in the first game.
In the preview for Amnesia: The Dark Descent (first called Unknown and dating back to May 15, 2009), the Terror Meter had a lower pitch.