Definition
A small movable marker on the face of an altimeter that the pilot can set to a chosen altitude as a visual reference. It does not change how the altimeter measures altitude; it simply provides a quick visual cue showing a target altitude such as a level-off, decision altitude, or minimum descent altitude.
Plain English
A little pointer on the altimeter that you can move to mark an altitude you want to remember, so you can see at a glance when you are reaching it.
Context Anchor
Seen on cockpit altimeters and altitude displays, especially during instrument flying, approaches, climbs, descents, and altitude changes assigned by air traffic control.
Derivation
In aviation, a 'bug' is any small movable marker placed on an instrument to flag a chosen value. The term comes from older mechanical usage where a tiny added piece on a dial was called a bug because of its small, attached appearance. Calling it an altimeter bug simply means the bug lives on the altimeter.
Why Pilots Care
Provides an immediate visual reference for assigned or minimum altitudes, reducing the need to read numbers constantly and supporting better altitude awareness.
Intuition Check
“Bug” does not mean a fault in the instrument here. It means the small marker you set on the altimeter to show a selected altitude.
Example Sentence 1
Before starting the approach, the pilot set the altimeter bug to the decision altitude as a visual reminder.
Example Sentence 2
With the altimeter bug at decision altitude, the pilot could monitor the descent without constantly glancing at the numbers.