According to my mother, a weed is a plant that is growing where you don't want it to grow.
But my personal favorite is from quotegarden by Doug Larson. A weed is a plant that has mastered every survival skill except for learning how to grow in rows.
One of the gardens on our tour has amazing weeds - they are actually cultivated plants (coral bells and lady's mantle) that pop up everywhere in his garden. They grow so thickly, there is no room for crab grass or dandelions to sneak their little seeds into the ground.
Of course, his garden is almost 40 years old and ours is only 5 years old, so there's a better base of cultivated plants at his place.
So, while my garden grows, I guess I'm stuck with weeding for now.
I wonder if the tour would appreciate a concrete garden???