Although the word 'weed' means something to almost everyone it is not easy to give a wholly satisfactory definition of this class of plant. Common definitions of weeds (Blatchley 1912; Harper 1944; Sykes 1978) include the very important and central idea that they are exclusively associated with people and their activities, and that they are undesirable and have a negative value.

A plant may be useful in some situations but a weed in others. For example, Bermuda grass (Cynodon dactylon) is quoted as one of the world's worst weeds but in some places it is better known as a valuable lawn grass or as a major fodder grass. Similarly, a crop plant which carries over into the next crop in a rotation is a weed.

The best simple definition of a weed is:

"Plants existing at places and/or times in which they are considered undesirable."