Real Dictionary

bulb
countable noun: A bulb is a root shaped like an onion that grows into a flower or plant.
countable noun: A bulb is the glass part of an electric lamp, which gives out light when electricity passes through it.

bulb in American English
an underground bud that sends down roots and consists of a very short stem covered with leafy scales or layers, as in a lily, onion or hyacinth
noun: a usually subterranean and often globular bud having fleshy leaves emergent at the top and a stem reduced to a flat disk, rooting from the underside, as in the onion and lily

bulb in British English
noun: a rounded organ of vegetative reproduction in plants such as the tulip and onion: a flattened stem bearing a central shoot surrounded by fleshy nutritive inner leaves and thin brown outer leaves