The Fungi imperfecti or imperfect fungi, also known as Deuteromycota, are fungi.
Which do not fit into the commonly established taxonomic classifications of fungi that are based on biological species concepts or morphological characteristics of sexual structures because their sexual form of reproduction has never been observed, hence the name imperfect fungi.
Deuteromycetes are called imperfect fungi because sexual stages are not observed in them, e.g. Alternaria, Colletotrichum, etc.