Why is the sporic meiosis life cycle referred to as an alternation of generations?

Some life forms alternate between a haploid generation and a diploid generation. Unlike most animals, including humans, where the haploid stage of life is restricted to single celled sperm and eggs, the life cycle referred to in this question involves both the haploid and diploid generation. Alternation of generations is common in the plant kingdom including such groups as mosses and ferns. The plant in the haploid generation is called a "gametophyte" and the plant in the diploid generation is called a "sporophyte".