A phylum in the kingdom Fungi, representing the largest of the major groups of fungi, and distinguished by the presence of the ascus, a specialized saclike cell in which fusion of nuclei and reduction division occur. The resulting nuclei are delimited by membranes through free-cell formation and a wall to form ascospores inside the ascus. In most ascomycetes, each ascus contains eight ascospores, but the number may vary from one to several hundred.