You can think of trill as one picture: something moving back and forth so fast that you stop hearing separate beats and hear a shake instead. In music that movement is between two notes next to each other. Outside music it can be a bird's throat, a tongue against the roof of the mouth, or a small electronic beep repeating. The speed is the point — slow the same movement down and it is just two sounds taking turns.
From the word’s history
The supplied Wiktionary entry traces trill through Middle English trillen to Italian trillo / trillare, the name of the musical ornament. That fits the shared picture: the word starts as the two-note shake and is then lent to any sound with the same fast back-and-forth quality.
來源資料指出，trill 經由中古英語 trillen，來自義大利文的 trillo／trillare，也就是音樂上的顫音。這和整個字的共同畫面對得上：它先是「兩個音之間的快速來回」，之後才被借去形容任何有同樣抖動感的聲音。
Wiktionary (kaikki extract supplied in this packet; source_ref kaikki:3f6f69a190036ae4:496840719:17714): “From Middle English trillen, from Italian trillo, trillare.”
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