Definition
Glass that has been strengthened by a controlled heating and rapid cooling process, which puts the outer surfaces into compression and the inner core into tension. This makes it several times stronger than ordinary annealed glass and causes it to break into small, relatively blunt fragments rather than long sharp shards.
Plain English
Glass that has been heat-treated to make it much stronger and to break into small, less dangerous pieces instead of long sharp ones.
Context Anchor
Seen in aircraft window, windshield, and glazing discussions, especially during inspection, repair, or replacement of transparent panels.
Derivation
From the Latin temperare, meaning to mix in proper proportion or to moderate. In metallurgy and glassmaking, tempering came to mean a heat treatment that brings a material to its proper, balanced strength. So tempered glass is glass that has been heat-conditioned to its strongest, most balanced state.
Why Pilots Care
If the glass breaks in flight or on the ground, the small granular pieces reduce the risk of injury and maintain some structural integrity until landing.
Analogy
Tempered glass is similar to the glass used in many car side windows: it is made stronger than ordinary glass, and if it fails, it tends to break into small pieces.
Intuition Check
Do not read “tempered” as “able to handle any temperature.” Here it means the glass has been heat-treated to make it stronger and to change how it breaks.
Example Sentence 1
The cabin side windows are made of tempered glass to reduce the risk of injury if a window is struck and breaks.
Example Sentence 2
During the preflight inspection the pilot noticed a star-shaped chip in the tempered glass of the windshield and reported it for maintenance evaluation.