Homecoming refers to the high school dance Peter attends in the film
(I haven't seen the film yet
so I'm trying to avoid spoilers for myself and others.)
A homecoming dance is a common high school tradition, and Peter, as a high school student, attends one during the film.
It is featured as the setting for the final act .
This is what the film's title refers to.
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This was addressed in an interview with the chairman of Sony Pictures, Tom Rothman.
[Homecoming is] relevant to the story because it takes place in high school and there is a scene — that I guess I can give a little thing — at ahomecoming dance.
I’ve actually seen the dailies of it because wealready did
the high school shooting of it in the first couple weeksand it’s fantastic.
But it’s also a homecoming to Marvel.
And ahomecoming to the cinematic universe that Spider-Man belongs in.
Soit’s a pretty unprecedented deal between two studios, but we’re reallyproud of it.
And all I can tell you is, those guys at Marvel — I thinkthe technical term is — they know their sh-t.
‘Spider-Man: Homecoming’
Title Explained By Sony Head Tom Rothman
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To add on to the previous answers, the homecoming dance is a major plot point in the film: At that critical scene, he makes the decision between his love interest, Liz, and stopping Vulture from his criminal activities.
But it's worse than that, because Vulture, not to be clichéd or anything, turns out to be Liz's father.
(To break the cliché, though, Liz doesn't know of either her father's or Peter's alter egos.)
It's because of that homecoming dance that Vulture finds out who Spiderman really is.
I'm sure someone out there is still trying to connect homecoming to the Winter Soldier command words: Longing, Rusted, Seventeen, Daybreak, Furnace, Nine, Benign, Homecoming , One, Freight Car.
Good morning, Soldier.
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The beginning of the movie has Peter return home after Spiderman's huge debut into the world of superheroes, both fighting with and against Avengers at the airport (Captain America : Civil War).
Instead of his world being completely changed, everything is essentially back to normal.
Before being drafted by Tony Stark, he had already been a bit of a minor vigilante and he returns to that when he fails to receive the "Avenger" missions he wants and expects.
At the end of the movie, he comes full circle and returns home again.
He rejects Tony's offer to become a full-time Avenger and returns home to be the friendly neighborhood Spiderman.
The Homecoming Dance came across to me as simply being a play on the word "Homecoming" rather than being the reason for the title.
Using any other semi-formal or formal dance would have made no change to the story.
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Another reason, in addition to the blatantly obvious ball, mentioned by others, is that after being fired from the "internship" with mister Tony Stark, he is welcomed back to Avengers at the end.
And don't forget the cliffhanger with aunt
May after coming home at the end ...
Also it could mean coming back to his roots (school, ordinary things, less superheroing) ... or all of the above.
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The out-of-universe reason looks obvious. Marvel celebrated the fact that it can work with Spider-Man again after it sold its movie rights to Sony.  But, if you look at other movies in MCU, names viz. The First Avenger, The Dark World, Ragnarok etc reflect in-universe things. What does  Homecoming  signify in-universe?
What's the in-universe relevance for the name of Spider-Man: Homecoming?
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<marvel><marvel-cinematic-universe><spider-man-homecoming><title-explanation>
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The word "Homecoming" refers to the high school dance that Peter attends in the film. It could also be a reference to Spiderman returning to his roots. The word Homecoming in the title refers to a number of things - Spiderman's return to the Marvel stable, Peter's return to his home neighbourhood and Peter's attendance at an actual homecoming dance.
The title refers to the high school tradition of the homecoming dance, as Peter attends one of these in the film. The title also refers to Spiderman's homecoming to Marvel. The title refers to the way Peter returns home after rejecting Tony Stark's offer to become a full-time Avenger and how life then returns to normality for him.