I'd always wondered about 'tea' for UK people. I always thought it was tea (duh) with maybe some cake or cookies or a few finger sandwiches. Just to hold you over until a late dinner.
To add to the confusion, my mid-western farm relatives (that we visited often) would have a fairly large meal (meat, potatoes, cooked vegetables) for the men who had been working hard all day. Then a lunch. Bigger meal. Then another meal around 3 p.m. Then supper at 5-6.
THEN before bed (which was early) a supper of warm milk, fresh from the cow, warm homemade bread, fresh from the oven to hold you over until breakfast at about 5 a.m. which was so huge I don't have the energy to type it all!
All this on a wood-burning stove by my 100 lb aunt! They don't make 'em like that any more.
I think the in-between meals were called supper too. 5 meals that would feed me for almost a week!!!
Ahh, the tea issue!
In the UK afternoon tea is taken in the middle of the afternoon and consists of small sandwiches, scones and cake. It began, so it's said, because society ladies found it difficult to go from a light lunch at midday to dinner at around 8pm without sustenance. It began as a social occasion for the leisured classes but now it's an occasional treat enjoyed by anyone when on a day out or on holiday or if you want to have a casual get together with (usually female) friends.
And then there is "high tea". This is further down the social scale. It's a "sit down" meal in the evening between 5-7pm involving a knife and fork main course - eg ham and salad or a hot fish, meat or cheese dish, followed by cake and/or bread and jam and the ubiquitous pot of tea. The average working man took a packed lunch to work unless his work place had a canteen serving meals. (And whether it was sandwiches or in the canteen it would have been referred to as "dinner".)
Dinner was an evening meal for the upper and middle classes until the second half of the last century. Dinner would consist of possibly a first course such as soup, then a main course and a pudding (general name for a sweet course). Gradually "dinner" has taken over from "(high) tea" in many families.
However, just to confuse the issue, Sunday, Christmas and school dinners are almost always at lunch time.
Supper isn't usually a meal unless you are getting home very late, say from a late shift at work or the theatre when it might be a sandwich or something on toast. Most people use "supper" to mean a hot drink and a biscuit before bed these days.
Of course, none of this is set in stone. For example in the post war period of social mobility there was a bit of a fad for people who had come to think of themselves as too posh for high tea but not posh enough for dinner, to call their evening meal "supper" but I don't think it's very much used in that sense these days.
There are certain references in the above that may sound a bit snobby but that wasn't my intention.