means Water released by the melting of snow, ice, or a glacier, especially the seasonal runoff that swells rivers and lakes.

from A transparent compound of plain Germanic stock: "melt," from Old English "meltan" (to dissolve or become liquid, a cousin of words for softening), plus "water," from Old English "wæter." The pairing is modern and scientific — the kind of literal term geologists and glaciologists build by bolting two honest words together — and it rose to prominence as the study of glaciers did.