Melody
Etymology
From Greek "melōidia," a joining of "melos" (song, tune — also, oddly, "limb") and "aōidē" (song, the root of "ode"). It reached English through Late Latin and Old French. The word has always meant song, but in music theory it narrowed to the single-line tune as opposed to its accompaniment.
noun · /MEL-uh-dee/
A sequence of single notes heard as a recognizable, singable line.
What it names
The horizontal, one-note-at-a-time tune you hum — the part of music memory latches onto — as distinct from the stacked notes sounding together.
Example
You could strip away every instrument and still whistle the melody of that chorus.
Related idea
Melody runs horizontally through time — one note after another — while harmony runs vertically, notes sounding at once beneath it. A tune is melody; the chords supporting it are harmony.