Iliad means "the poem of Ilium". Ilium, or Ilios, is the other name of Troy, the city of King Priam, and the title is formed in Greek with the ending -as, -ados, which indicates belonging: the Ilias is the song about Ilium. The title is not the poet's, who opens the poem by announcing the wrath of Achilles and not the city. It is the name by which the Greeks were already citing the poem in the fifth century BC, Herodotus among them.
In the poem the two names live side by side. Agamemnon foresees in Book IV: "the day will come when sacred Ilium perishes, and Priam, and the people of Priam". Hector repeats the sentence almost word for word to Andromache, in Book VI, and the name of Troy appears alongside it, for the city and for the region.
The title is, at bottom, a little misleading. The Iliad does not tell the story of Troy or of its fall: it tells a few weeks of the tenth year of the siege, around the wrath of Achilles, and ends with the funeral of Hector, with the city still standing.