A geophyte is a plant that depends for part of its life cycle on an underground reserve of food, that is a bulb, corm, rhizome, or tuber. In adverse conditions, such as the cold of winter, the above-ground growth of the plant can die back and the geophyte enters its so-called dormant phase, though below ground it is still active, not dormant.
The storage organ is often referred to as a bulb, regardless of its exact shape and which tissues it is derived from, but the true bulb of the lily, onion, hyacinth, daffodil, tulip and so on is formed from modified leaves around stem tissue. The leaves, called scales, are the storage medium.
A corm is where the stem itself is modified: examples are the crocus and gladiolus.
Both corms and bulbs arise upward out of a compacted base of stem material called a basal plate. Tubers and rhizomes are also made from stem material, but grow in different ways: a tuber goes downward, like a potato, and a rhizome grows sideways.
When the root itself is enlarged to store food, as in the dahlia and anemone, it is called a tuberous root.
The name geo-phyte means earth-plant.
http://virtual.clemson.edu/groups/hort/sctop/geophyte/type.htm