Sub

Sub is a versatile shortening and morpheme used across many domains. As a standalone noun or adjective, it often signals something beneath, secondary, or auxiliary. It also functions as a prefix in forming words and phrases.

In naval contexts, "sub" is a common shorthand for submarine, a watercraft capable of operating underwater. Submarines have roles in defense, underwater exploration, and rescue. Modern examples include ballistic missile submarines and attack submarines.

In sports and broadcasting, "sub" denotes a substitute. A player may be substituted into a game; in soccer, the coach can "bring on" a sub. In classrooms or meetings, a "sub" may refer to a substitute teacher.

In audio systems, "sub" refers to the subwoofer, a speaker dedicated to low-frequency sounds (typically 20–200 Hz). Subwoofers enhance bass in music, cinema, and car audio.

In online communities, especially Reddit, "sub" is short for subreddit, a user-created forum focused on a topic. Subreddits have unique rules and communities; users subscribe to them to customize their feed.

As a prefix, "sub-" means under or below, and appears in many words such as submarine, subterranean, subtitle, substandard, and suboptimal.

In North America, "sub" is also a common shorthand for a submarine sandwich, a long roll filled with meats, cheese, and toppings.