A young eel, especially one in the transparent larval stage that swims upstream from the ocean into rivers for the first time.
From Middle English, possibly deriving from Old English origins related to 'eel,' with '-er' as a diminutive or agent suffix indicating a young or small eel.
Elvers are transparent like glass when they first enter rivers, and European countries have actually built special 'eel passes' around dams because millions of elvers need to reach freshwater to grow—it's one of nature's most determined migrations.
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