What is a weed anyway?

Any plant can be a weed, it just depends on the local circumstances.  There is no special biological attribute that makes a plant a weed.  Weeds are just plants we (humans) do not want at that particular place.  This could be because we are trying to grow something else, that we think they look ugly, they might be poisonous, the might take over and crowd out other plants and change the ecosystem habitat, or we might think they are just wrong.

Weeds are a human mental construct - without human agriculture and horticulture or dwellings on earth there wouldn't be any weeds.  There would just be plants...

So, in our rather selfish pursuits we tell the plants that like to live in 'our' places, that they are unwanted (=weeds) and should be eradicated.

Some definitions:

Merriam-Webster:

A plant that is not valued where it is growing and is usually of vigorous growth; especially one that tends to overgrow or choke out more desirable plants.

The Free Dictionary:

A plant considered undesirable, unattractive, or troublesome, especially one growing where it is not wanted, as in a garden.

Dictionary.com:

A valueless plant growing wild, especially one that grows on cultivated ground to the exclusion or injury of the desired crop. Any undesirable or troublesome plant, especially one that grows profusely where it is not wanted.

Biology Online:

Any plant that is growing in a place where a human wants a different kind of plant or no plants at all.

Encyclopedia Brittanica:

Any plant growing where it is not wanted.

Oxford Dictionaries online:

A wild plant growing where it is not wanted and in competition with cultivated plants.