Definition
A small, movable marker fitted to the face of a flight or engine instrument, used by the pilot to set and visually reference a specific value such as a heading, airspeed, or altitude.
Plain English
A little pointer you slide around the edge of an instrument so you can see at a glance the number you want to fly, like a heading or a speed.
Context Anchor
You will see bugs on cockpit displays and instruments when pilots set assigned headings, target speeds, selected altitudes, or other desired values.
Derivation
From the older mechanical sense of 'bug' meaning a small attached device or fitting. Calling the marker a 'bug' captures the idea of a tiny extra piece clipped onto the instrument face.
Why Pilots Care
Provides an immediate visual cue for target speeds or headings, reducing the need to read exact numbers and lowering pilot workload in critical phases of flight.
Analogy
A bug is like sliding a small bookmark to the number you want to remember, except it is placed on a flight instrument instead of a page.
Intuition Check
Do not assume bug means an insect or a computer problem here. In this aviation use, a bug is a movable marker used to show a selected target value.
Example Sentence 1
Before turning onto the new heading, the pilot set the heading bug to 270 degrees.
Example Sentence 2
With the heading bug aligned to the runway course, the pilot maintained the assigned heading more easily.