Boil/bɔɪl/ Definition 1 of 5Pro Tip 1/2Visible Bubbles Look for active, visible bubbles forming in the liquid — a key sign it's boiling. You'll see bubbles rising when the water reaches a boil. Pro Tip 2/2Continuous Heat A liquid needs to maintain high heat to keep boiling — not just to start. Keep the burner on high to maintain the water's boil.