Brunch is a late-morning or early-afternoon meal that combines elements of breakfast and lunch. The word is a portmanteau of breakfast and lunch, and the meal is often more leisurely and social than a weekday midday stop—especially on weekends.
Lunch is the regular midday meal that typically follows a separate breakfast. It is usually timed around noon and functions as a practical midday refuel, whether the foods overlap with brunch dishes or not.
Source: Brunch — Wikipedia