Subwoofer
Definition
A loudspeaker designed to reproduce very low frequencies, typically below 80–120 Hz. The ".1" in surround formats such as 5.1 and 7.1 refers to the LFE (low-frequency effects) channel routed to the subwoofer.
In Simple Terms
A speaker dedicated to deep bass—the frequencies you feel more than hear. In surround and Atmos setups, it handles the '.1' channel: explosions, rumble, and low-end impact.
In Practice
The impact of an explosion in a film is felt physically through the subwoofer, which handles low-frequency energy below what the satellite speakers can reproduce.