Underground vertical shoots that have modified leaves (or thickened leaf bases) that are used as food storage organs by a dormant plant, e.g. onion, daffodil, tulip. Bulbs often function in reproduction by splitting to form new bulbs or producing small new bulbs. Bulbs are a combination of stem and leaves, with the leaf portions making up most of the bulb.
Definition adapted from URL: http://en.wikipedia.org/wiki/Bulb