Dinner, plain and simple.
Sup. That’s certainly the word that’s used in various nineteenth century novels. I have a friend who likes to ask “will you sup with us tonight?”
Born and raised in Yorkshire, and I call it ‘tea’.
The meals go -
Breakfast
Dinner
Tea
Supper is a snack before bed. But I never have one.
From the Shallow South (Kentucky):
I use “dinner” and “supper” interchangably for the evening meal, probably in equal proportions.
The mid-day meal is “lunch” unless it’s a special-event sort of meal, like Thanksgiving or Easter or when we’d go to my great-grandmother’s to eat after church on Sundays; then it’s “dinner”. Many older people where I grew up would refer to most any mid-day meal as “dinner”. I would never refer to a mid-day meal as “supper”.
I don’t think I ever use the term “supper” without repeating what someone else may have already said. When referring to the evening meal, I always call it “dinner”, and I always call the midday meal “lunch”. ;j
At home it was always (& still is when I visit)
Breakfast
Lunch
Tea (which really is just a mug of tea and a biscuit not a ‘meal’)
Supper
The reasoning - slightly ‘posh/old fashioned’ parents (we also have a lounge rather than a sitting/living room) and timing I think - we had to wait for Dad to get home before we had our evening meal so often didn’t eat until 8, 8.30 or even later so i guess it was more ‘supper’ time.
Friends, North Notts/ South Yorkshire & Wales, either say tea (meaning a meal) or dinner and eat between 6 and 7.
Now I naturally tend to say things like “You’ll have to come over for a meal one evening” or “What do you want for food?” “What do you want to eat?” Possibly this is my subconscious making sure I avoid sounding pompous (witness some of the reactions to ‘supper’ in this thread) or feeling uncomfortable (I’m just not used to saying ‘dinner’).y
Breakfast
Lunch
Dinner - I can’t recall using the word supper myself, but I know it’s here in the day
Which is complicated by the fact that my mother (but not my father) grew up with:
Breakfast
Dinner/Lunch
Supper
After several years of trying to convince the rest of us that dinner can be used in place of lunch, she no longer is confused when we ask about “dinner.” Maybe the beef and pork campaigns made her see the light 
I’ve lived in the Northeastern United States my whole life. It’s always dinner and not supper. I can’t even use the word without a slight giggle and the feeling that I’m being vaguely pretentious.
I have never used the word supper. It’s breakfast, lunch, and dinner. Snacks in between.
When I hear someone use the word supper, I think Margo Channing (Bette Davis) in “All About Eve.” Or some other Broadway type setting – After the show we’ll go to Sardis for supper and wait for the reviews.
Or a Midwestern farm family straight out of a Norman Rockwell painting having a big dinner in the middle of a Sunday afternoon. They would call the meal supper.
I use the word “dinner”, and so does everyone else in my family.
Northern California, with family members from Utah (paternal grandfather and grandmother) and upstate New York (mother).
Dinner
[pippin]What about second breakfast?[/pippin]
I call it dinner.
Growing up it was always supper. Now I use either interchangably, but probably say dinner more often.
Dinner…now, and when I grew up.
Oddly, if I was threatened with punishment, the threat would be, “You’ll go to your room without supper, mister!”
That was the only time I heard ‘supper’, and I’m betting my parents were quoting some cheezey TV mom/dad.
Raised in Maryland, like both my parents.
Nobody in my immediate family ever uses the word “supper”. It goes:
Breakfast
Lunch
Dinner
If supper were used, it would be interchangeable with “dinner”–largely because this is the way my grandfather (raised in Massachusetts) uses it when he’s around.
Raised in rural Georgia, but my answer is the same. Well, except for “tea,” which is a sweet, icy drink and not a meal.
“Lunch” has crept into my vocabulary. My parents always used “dinner” for the midday meal, but I am apt to call the midday meal “lunch” if it is a light or quick meal, “dinner” if it is more elaborate.
I don’t know where I got my reasoning on this topic, but in my mind:
The midday meal is always called “lunch”.
Supper is modertate-sized meal eaten in the early evening (between 5 and 7). Very casual; not everyone in the house is expected to come to the table and eat together for supper.
Dinner is a large, formal meal eaten in the evening (as early as six but sometimes 10 or later). Must be eaten with other people; if you’re alone, you’re having supper.
It’s mainly a matter of what kind of vibe you get from the meal. Say you’re hanging out with some friends, just driving around and thinking of going to see a movie. You all decide to have a bite to eat before the movie. If you just grab some fast food, that’s supper (even if you eat it together). If you go to a restaurant for steaks, that’s dinner.
Hey, it makes sense to me.