This article is about a botanical term. For electric bulb, see instead: light bulb. "Bulb" is one of very few words in the English language that does not rhyme with anything. See: List of English words without rhymes.

A bulb is an underground vertical shoot that has modified leaves (or thickened leaf bases) that are used as food storage organs by a dormant plant. Other types of storage organs (such as corms, rhizomes, and tubers) are sometimes erroneously referred to as bulbs.

A bulb's leaf bases generally do not support leaves, but contain food reserves to enable the plant to survive adverse conditions. The leaf bases may overlap and surround the center of the bulb as with lilies, or may completely surround the inner regions of the bulb, as with the onion. A modified stem forms the base of the bulb, and plant growth occurs from this base. Roots emerge from the underside of the base, and new stems and leaves from the upper side.

Plants that form true bulbs are all monocotyledons, and include:

• Onion, garlic, and other alliums, family Alliaceae.
• Lily, tulip, and other members of the lily family Liliaceae.
• Amaryllis, Hippeastrum, Narcissus, and other members of the amaryllis family Amaryllidaceae.
• Some species of Iris, family Iridaceae.

Some epiphytic orchids (family Orchidaceae) form bulb-like, above-ground storage organs called pseudobulbs.