Long benches with no back support where crowds of people sit to watch sports games or other outdoor events.
From 'bleach' (to make white by sunlight) because these benches were originally painted white or made from bleached wood. They became bright from sun exposure.
The original bleachers were literally exposed to full sunlight which would bleach (lighten) the wood, and sitting in them would tan or bleach your skin—so they were also a status symbol showing you had time to sit in the sun!
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