The technical definition of a weed plant is one that is where it is not supposed to be. Few people would regard an oak tree seedling as a weed, but if it is growing in the middle of a perennial bed, it is indeed a weed. The same lawn grass plant that gets watered, trimmed and fertilized when in the lawn will summarily be ripped from the ground in a vegetable garden and tossed into the compost pile.
There are some plants that are weeds regardless of where they appear. Poison ivy and kudzu are two examples. Some other plants can do quite well where they have been planted on purpose, but spread seeds to other parts of the landscape where they are not welcome at all. Wisteria is a prime example. As it prettily decorates a fence or an arbor, it also spreads quick-growing seeds throughout nearby lawns and garden beds.
Pale Yellow Platter - A Weed Macro (Photo credit: cobalt123)
Thus a weed may not always be worthy of herbicide if it can have value in a better location. Even dandelions have value for some people, although far greater numbers of people regard it as a weed. Other than when the subject is a noxious weed, the firm definition of what is a weed is confined to individual areas of specific landscape applications.