Definition by
Webster: "a usually uncovered stand of tiered planks providing seating for spectators".
That's the definition. And while you might think there's some cool story about how bleachers got to be called bleachers, it's really not all that cool. While the rows in the bleachers are constructed from aluminum nowadays, in the 19th century they were made out of wood, which was bleached by the hot sun beating down every day. In fact, that was the standard way to bleach wood - keep it under the sun for extended periods of time.
The first recorded use of the word "bleachers" is from the Chicago Tribune on May 6th, 1889: "The grand stand and bleachers were well filled with something over 2000 spectators."