What is the definition of a "protist"?

According to About.com, organisms are classified into kingdom Protista by exclusion, when they are eukaryotic, meaning they are made up of cells where the nucleus is surrounded by a membrane, but cannot be otherwise classified as animals, plants or fungi.

How Stuff Works adds "This kingdom, sometimes called Protoctista, consists of single-celled micro-organisms and simple multi-celled organisms." Most of the protist phylums make their home in water, being considered marine or freshwater phylums, although a few are terrestrial or parasitic. A very diverse set of organisms is represented, including amoeba, kelp and various kinds of algae, like Irish moss, which is a kind of red algae.