In broad terms, a holotype is the body of a dead animal preserved as the official example of a species.
A holotype is the single example (either a physical specimen or an illustration) that is used to define a taxon (such as species or sub-species). The holotype is set when a taxon is formally described, and is generally preserved in a museum (for physical specimens) or a formal publication (for illustrations).
Ideally, a holotype is typical member of a species, but this is not always possible. Holotypes of extinct species may consist of a fossil, a fossil fragment, or other preserved tissue. Zoology is not a fast-moving field, and the holotype is sometimes not the best example, but only the first one sufficiently described. If the original holotype is officially replaced, the new exemplar may be referred to as a neotype.
On rare occasions there may be more than one holotype; in this case each example is called an isotype. Examples of a species that are not the holotype may be referred to as paratypes. Sometimes a counterpart of the opposite sex to the official holotype is designated as an allotype, essentially being a second holotype used to formally describe sexual differences.