I tend to think of it as mastery of the specifically musical manner of speech. (And, incidentally, I suspect that language partakes of music more than vice versa.) It's not just the ability to compose good melodies, but the ability to write melody as naturally as one might speak. In Tchaikovsky, this manifested itself in the ability not only to write good tunes, but to write ones so good as to induce awe. (Just now I am thinking of the Sleeping Beauty waltz, and the way he extends the syncopated phrases that end each section of the main tune.) The Swan Lake waltzes are favorites of mine also. Tchaikovsky is in fact a "Waltz King" if there ever was one. On the topic of "Waltz Kings," Lehár is considered one, but I have to admit that Strauss of Blue Danube fame has more claim to the title. After listening to this concert-waltz for a few dozen bars, one starts wondering: "Where the hell is he finding all these fantastic tunes?" And then he spins out more and more of them--and they all sound like they've been around forever. (Ayn Rand, who said she hated this composition, obviously at the time of her writing had not reached a full appreciation of this quality of melody.)