oxalic acid

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

• noun A poisonous, colorless crystalline dicarboxylic acid, C₂H₂O₄, found in many plants, such as spinach, and used as a bleach and rust remover.

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

• noun organic chemistry A colourless, crystalline dicarboxylic acid, (COOH)2, found in rhubarb, spinach and other plants, often as crystals of the calcium salt.

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

• noun a toxic colorless crystalline organic acid found in oxalis and other plants; used as a bleach and rust remover and in chemical analysis