Billy Pilgrim's name in Vonnegut's Slaughterhouse Five has definite symbolic and connotative meanings.

We can look at Billy as a common name for a common man that represents all men. We can also look at Billy as a child's name—a nickname or diminutive form of William. This interpretation is much more interesting because it connects to the subtitle of the novel, "The Children's Crusade." With the novel's autobiographical nature and Vonnegut's concern with the idea that it was almost literally innocent and unprepared children who fought WWII, this symbolic meaning to the name is illuminated. Billy Pilgrim was wholly unprepared for war: his uniform did not fit and he was almost immediately captured.

As for his surname, it seems to fit that he is a traveler. Pilgrims are known for leaving their homes for a definite purpose. Chaucer's pilgrims in The Canterbury Tales were seeking renewal and atonement at a holy site. Plymouth Rock's pilgrims were seeking freedom from religious persecution across an ocean. Vonnegut's "Pilgrim" goes from New York to Dresden in Germany and back, and then to another planet, Tralfamadore, all in a frenetic search for meaning.

The child traveler returns from his travels with a Serenity Prayer and with odd beliefs regarding time and experience. He does not, however, come closer to finding real meaning. 

Billy Pilgrim's last name is an allusion to John Bunyan's Pilgrim's Progress but also refers, more universally, to Billy's role as a pilgrim, a person on a spiritual journey. Billy is an optometrist, a symbol of his role as one who sees. He enters the army as an assistant chaplain and at first has a conventionalized Christianity that most of the soldiers reject. After he is shattered by witnessing the bombing and utter destruction of Dresden, he eventually does go on a journey, kidnapped by the alien Tralfamadorians, who bring him to their planet. This leads him to a spiritual transformation. The Tralfamadorians show him that the past, present, and future are all one, strung out like a mountain range, so Billy learns to live in the present, to not fear death, and to adopt the phrase "so it goes" to signal his acceptance that life is what it is. 

His first name, Billy, is a very common, everyday name, but Vonnegut is ever the jokester, so the name possibly refers to slang for speed, an amphetamine that might send one on a "trip," something completely appropriate to the counter-cultural drug culture of 1969, the year of the book's publication, and to Billy's interplanetary journey.

Ilium has a double meaning. Ilium really is a city in New York, home to the Rensselear Polytechnic Institute. Since the novel explores the role of technology, good and bad, it is not surprising that Vonnegut placed his protagonist, who witnessed the destructive effects of technology on Dresden, in a city housing a technical institute. Ilium is also the Trojan city completely destroyed by the Greeks, another reference to the destruction of Dresden and to the human capacity for violence. 

"Billy" is a common name, one of the most common male name in the English language. By naming his protagonist Billy, Vonnegut is identifying him as an everyman, a character with whom readers can identify. Our experience is heightened by believing that he is an "average joe", just like us. Readers can put themselves in his shoes as he journeys.

The surname "Pilgrim" is reference of Billy's role in the novel. He is the time-travelling pilgrim, seeking truth and peace, like so many pilgrims who have come before him. It is a spiritual and intellectual journey he is on throughout the novel, coming to a sad truth of helplessness in the end.

Vonnegut places Billy in Ilium as reference to Troy, the city made almost mythical through Homer's epic poem, the Illiad. The characters in the Illiad, the historical figures that mix-in and interact with the mythical and divine, are larger than life. The adventure is larger than life. So, therefore, is Billy's adventure. By using this allusion, Vonnegut wants us to universalize Billy's experience and see the allegorical meanings associated with the protagonists path. Like so many characters in the Illiad, Billy is at the mercy of forces beyond his control.