A weed is a plant growing in a cultivated area that wasn't planted or
intended by the farmer.
So a corn plant growing in a soybean field is a weed and arugula
growing in the spinach patch is a weed.
Modern agriculture uses very strong and toxic herbicides to eliminate
these plants growing in the wrong place. Sometime only a few onces
per acre will eliminate all but the desired crop.
I don't get too hung up on weeds.