noun
BAD : She usually has lunch at a snack in Barceloneta Square.
GOOD : She usually has lunch at a snack bar in Barceloneta Square.
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snack = a light meal or something that you eat between meals: 'Instead of going out to lunch, I usually have a quick snack in my office.'
snack bar = a café or similar place where you can buy a light meal: 'The snack bar gets very busy at lunchtime.'