A species that satisfies one major criterion of the scientific definition of species - it is reproductively isolated from others - but which is morphologically indistinguishable from one or more others. The group of those indistinguishable species together form a cryptic species complex. The individual species within the complex can only be separated using special techniques, like molecular genetics such as DNA barcoding.
Links: A cryptic species complex in the ant parasitoid Apocephalus paraponerae (Diptera: Phoridae).