1. a body of water with a calm, smooth surface; water without waves or significant current
Usage: commonly used in paddling and water sports contexts
2. a discipline in competitive paddling or rowing performed on lakes, ponds, or other non-moving water
Usage: sports terminology

Examples

• The kayakers preferred flatwater for their morning practice.
• She trained in flatwater events before competing in the national championships.
• The lake provided perfect flatwater conditions for the canoe race.
• Flatwater paddling requires different techniques than whitewater navigation.
• The pond's flatwater made it ideal for beginners learning to paddle.