Today, dinner almost universally refers to the evening meal. But it has had a long etymological history to get to that point.
Those with older relatives might have noticed them say “dinner” to refer to the midday meal—what we would usually call “lunch” today. It’s rather archaic today, but it used to be the dominant usage.
It comes to modern English from Old French disner (via Middle English dyner), which originally meant “breakfast”, but later meant “lunch”. Disner is evolved into modern French dîner, suggesting the same more recent history has taken place in that language as in English.
Disner comes, ultimately, from Latin *disiūnō, meaning “to break the fast”.
So, depending on when you are, “dinner”, and its etymological ancestors, could have meant breakfast, lunch, or dinner.
In the UK dinner is still typically the lunch meal, and evening meal is called tea.
Midlander here. Dinner is the “main meal” whenever it’s eaten. Tea is always the evening meal and lunch is always the midday meal though.
Not across all of the UK, definitely not here in the south, Me and a fellow midlander once spent a week trying to explain dinner = midday/tea = evening to someone at work, I think they’re still confused.
I saw the title of the post and my first thought was all of those options are still true in the UK depending on where you ask, lol.