See also: Metamorphism (homonymy)

The principal characteristic of a metamorphic virus is its capacity to modify its internal structure as well as the instructions which compose it. The metamorphism is a mechanism of defense used by different malevolent programs in order to avoid their recognition and their detection by protection and checking routines such as the software antiviruses.

A virus known as metamorphic does not contain a decoder nor a constant viral body, but is able to create new generations of itself different with each replication while preventing that a generation either too resembling with that which precedes it.