An onion (Allium cepa) is a pungent, edible bulb vegetable from the amaryllis family, known for its strong flavor, layers, and sulfur-rich compounds that cause tearing when cut. They grow hollow green leaves from a flattened stem, forming a layered bulb that can be red, yellow, or white, with shapes and sizes varying by variety, and are a staple in global cuisine for adding depth to dishes.