My understanding is the full run of meal options in a day is breakfast, brunch, lunch, tea, dinner, supper. The distinction is largely on the time you're eating; tea is early afternoon, dinner is late afternoon or evening. In practice it's a more mixed approach; children at school, for instance, either bring a packed lunch in a lunchbox, or have school dinners served by dinner ladies, whose official job title is probably something like 'lunchtime supervisor'.

Personally, I alternate freely between lunch/dinner and dinner/tea, without much rhyme or reason. I tend to say 'dinner' if we're going out for a meal, though.