Hellion's suggestion (seconded and ably expanded by NVZ) that meal covers any of the three main occasions for taking food during a usual day, and that snack covers any occasion for taking food at times other than those identified as meals, seems commonsense and well conceived. The only place where that classification might run into trouble is in figuring out how to handle a fourth regular occasion for taking food, as with tea time: is tea a meal or a snack or something else when it occurs daily and involves such comestibles as cucumber-and-watercress sandwiches and cake? I'm inclined to call it a "light meal" (as breakfast, lunch, and supper can be in some circumstances). Merriam-Webster's Eleventh Collegiate Dictionary (2003) goes so far as to define high tea as
a fairly substantial late afternoon or early evening meal at which tea is served
As for a single word that comprehends "heavy meal," "light meal," and "snack," there isn't a very satisfactory one, as far as I can tell. But one that you might press into service is repast, which is a fairly common word and yet doesn't have as narrowly fixed a meaning as meal. Here is the Eleventh Collegiate's definition of repast:
repast n (14c) 1 : something taken as food : MEAL 2 : the act or time of taking food
From these definitions, it appears that a feast, a banquet, a regular meal, a light meal, and a snack all qualify as repasts. Elsewhere (in its definition of the far less common word refection), the Eleventh Collegiate offers this interesting equivalence: "food and drink together : REPAST." According to that definition, a glass of Coca-Cola and a packet of crisps is a repast.