Definition
An indicator light assembly with a built-in pushbutton that, when pressed, completes a circuit to illuminate the bulb directly from aircraft power, allowing the technician or pilot to verify that the bulb itself is functional independent of whatever system normally drives it.
Plain English
A warning or indicator light you can press like a button to check that the bulb still works, without having to trigger the actual fault it's meant to warn about.
Context Anchor
Seen on aircraft warning, caution, or status light panels during preflight checks and maintenance inspections.
Derivation
The name describes its function literally: 'press to test.' The fixture is the housing that holds the bulb and the test switch in one unit.
Why Pilots Care
Verifies that critical warning and status lights will actually illuminate when needed, preventing missed alerts that could affect flight safety.
Intuition Check
Do not assume pressing the light proves the whole aircraft system is working. It usually proves only that the light and its basic lighting circuit can illuminate.
Example Sentence 1
During the preflight check, the pilot pressed each annunciator's press-to-test light fixture to confirm all warning bulbs were operational.
Example Sentence 2
The technician replaced the bulb inside the press-to-test light fixture after it failed to illuminate during the functional check.