Real Dictionary

weed
countable noun: A weed is a wild plant that grows in gardens or fields of crops and prevents the plants that you want from growing properly.

weed in American English 1
any undesired, uncultivated plant, esp. one growing in profusion so as to crowd out a desired crop, disfigure a lawn, etc.
to remove the weeds from (a garden, lawn, etc.)
to remove weeds, etc.
noun: a valueless plant growing wild, esp. one that grows on cultivated ground to the exclusion or injury of the desired crop

weed in American English 2
a garment
noun

weed in British English 1
noun: any plant that grows wild and profusely, esp one that grows among cultivated plants, depriving them of space, food, etc
verb: to remove (useless or troublesome plants) from (a garden, etc)

weed in British English 2
noun: a black crepe band worn to indicate mourning