There are countless definitions of weeds. Farmers might have their own definition - a weed is any plant that interferes with profit, maybe? For gardeners, weeds are plants that are "out of place". A poets definition of a weed is "a plant whose virtues have not yet been discovered" (Ralph Waldo Emerson, again).
The latest research shows that weeds benefit far more than crop or garden plants from the changes in CO2, making them more hard to control. In the future, we might all have to become poets in our attitude towards weeds.