"Wild" is a term that refers to a situation where a virus or worm has been released to where it is able to infect computers. Viruses and worms released into the wild are the only ones able to cause damage.
Non-wild or "Zoo" viruses and worms are confined to laboratories or hobby computers and networks. Often, antivirus companies create them to test their product's ability to detect new threats or to create a general detection for a piece of malware that has many variants. Proof of Concept viruses and worms, such as those created by ethical groups or writers like 29A or Gigabyte are also rarely released into the wild.