adj: growing where the natural vegetation has been disturbed; growing in rubbish, poor land, or waste.
noun: a plant growing in poor or disturbed soil.
Etymology: from Latin rudus, rubble. As an adjective, the word is listed as being in use since the mid 1800s; as a noun it's listed as being in use since about 1928.
Ruderal communities often form where soil has been disturbed by human activity (roadsides, abandoned fields, etc.), but the word also applies to communities that form after natural disturbances such as wildfires.
Chicory used to be a common ruderal species where I live, but populations have dropped in recent years (I'm not sure why).