A correspondent writes as follows: 'When I came across nouns for meals in my teaching I faced a problem because I belong to a non-Western culture. Dinner is the main meal of the day, I believe, in Western societies. It is usually eaten in the evening. What about lunch? It is usually a midday meal and people often have it at work. In Eastern societies, however, the main meal is usually in the afternoon [like lunch]. So, should we call it lunch or dinner?'
Dictionaries define dinner as a main meal, and leave open the question of time of day. This is because there is a great deal of regional and social class variation. In some parts of the UK (and also in some other parts of the English-speaking world) when people take their main meal in the middle of the day (eg in some farming communities) then that is called dinner. They wouldn't use the word lunch at all; and for them an evening meal would be called tea or more likely supper.
My own usage has changed over the years. When I lived in Liverpool, as a teenager, the meal I had in the middle of the day, whether at school or at weekends, was called dinner. Today, after a few decades of eating in the south of England, I call this lunch.
The standard word for a meal taken in the middle of the day is lunch. For many people, lunch is a light meal - it might only be a sandwich. Note that there are several ways of expressing variation at lunchtime, such as light lunch, big lunch, heavy lunch, and working lunch, which don't usually apply to the word dinner. After some publisher's lunches I've attended, I don't feel like eating for a week.
As always, there are exceptions. In schools, the traditional phrase for the lunchtime meal was school dinners and the people who serve it were called dinner ladies - and still are, in many places. And on Christmas day, most people sit down to a Christmas dinner - at lunchtime.
The two elements influencing usage, therefore, are the importance of the meal and the time of day. The main factor seems to be the importance of the meal, and so dinner would seem to be the solution, in the case of my correspondent. But he needs to listen out for the way others use the terms in English in his part of the world, for that will be more important than anything.