milk thistle

from The Century Dictionary.

• noun A thistle-like plant, Silybum (Carduus) Marianum, native in southern Europe, somewhat cultivated and spontaneous elsewhere. The leaves are variegated with white. Sometimes called lady's thistle.

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

• noun Any of the thistles in the genus Silybum.

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

• noun any of several Old World coarse prickly-leaved shrubs and subshrubs having milky juice and yellow flowers; widely naturalized; often noxious weeds in cultivated soil
• noun tall Old World biennial thistle with large clasping white-blotched leaves and purple flower heads; naturalized in California and South America