Melody, air, tune all denote a clearly distinguishable succession of rhythmically ordered tones.
Melody stresses the sweetness or beauty of sound produced by such an arrangement of tones. It also commonly suggests expressiveness or moving power and a carefully wrought pattern. Technically, as applied to complex musical structure, melody implies a contrast to harmony; it designates that kind of musical beauty produced by a continuous series of tones in one or more of the voice parts, in distinction from that produced by simultaneously sounded tones in all the voice parts.
Air is applied technically to the dominating melody, usually carried by the upper voices (as in a chorale or part-song). In more general use air is often applied to an easily remembered succession of tones which identifies a simple musical composition (as a song, a ballad, or a waltz) and which is more commonly and more precisely called tune; thus, one may refer to the air, or the tune, of a song.
Tune is also applied to the musical setting of a text (as a ballad, psalm, or lyric) <a hymn tune > and to a simple composition whether unison or harmonized.