"A weed is a plant that has mastered every survival skill except for learning how to grow in rows" -Greenville Master Gardener's Newsletter.
"Any plant whose virtues have not been discovered" - Emerson
"A plant that grows where it's not wanted" - a convential wisdom, as listed on Wikipedia
"Any plant that is not valued by the human society" - National Park Service
"A plant out of place" - Diane Relf, Extension Specialist