Picture two things standing at different heights and one line drawn across them. An equalizer is whatever brings the low side up until both sit on that line. It can be a goal, a fact of life that nobody escapes, a slider that lifts one part of a sound, or a bar inside a machine. In every case the job finishes at the same moment: when the two sides are level.
From the word’s history
The word is built in three steps: equal, then equalize (to make equal), then -er (the one that does it). So an equalizer is simply the thing that makes things equal.
這個字分三層長出來：先是 equal（相等），再是 equalize（使相等），最後加 -er（做這件事的人或東西）。所以 equalizer 字面上就是「把東西弄成一樣的那個」。
Wiktionary etymology tree supplied in this evidence packet (kaikki:3f6f69a190036ae4): "From equalize + -er (nominal suffix)."
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