Real Dictionary

rhizome
countable noun: Rhizomes are the horizontal stems from which some plants, such as irises, grow. Rhizomes are found on or just under the surface of the earth.

rhizome in American English
a creeping stem lying, usually horizontally, at or under the surface of the soil and differing from a root in having scale leaves, bearing leaves or aerial shoots near its tips, and producing roots from its undersurface
noun: a rootlike subterranean stem, commonly horizontal in position, that usually produces roots below and sends up shoots progressively from the upper surface

rhizome in British English
noun: a thick horizontal underground stem of plants such as the mint and iris whose buds develop new roots and shoots