Is this a plant or weed?

A weed is in a general sense a plant that is considered by the user of the term to be a nuisance, and normally applied to unwanted plants in human-made settings such as gardens, lawns, or agricultural areas, but also in parks, woods and other natural areas.

More specificallym the term is often used to describe plants that grow aggressively, and is in an undesired place.

source for above: http://en.wikipedia.org/wiki/Weed

"Weed" is also a slang term for marijuana.