celery

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A biennial European plant (Apium graveolens var. dulce) in the parsley family, having edible roots, leafstalks, leaves, and fruits.
• noun The crisp thick leafstalks of this plant.
• noun The seedlike fruits of this plant used as a flavoring.

from The Century Dictionary.

• noun An umbelliferous plant, Apium graveolens, a native of Europe, and long cultivated in gardens for the use of the table.
• noun See Vallisneria.
• noun On the Pacific coast, an aquatic umbelliferous plant, Œnanthe sarmentosa, the stems of which have the taste of celery and were eaten by the Indians. The poisonous Oregon water-hemlock is said to be sometimes mistaken for this.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Bot.) A plant of the Parsley family (Apium graveolens), of which the blanched leafstalks are used as a salad.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A European herb (Apium graveolens) of the carrot family.
• noun uncountable The stalks of this herb eaten as a vegetable.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun widely cultivated herb with aromatic leaf stalks that are eaten raw or cooked
• noun stalks eaten raw or cooked or used as seasoning