An online identity used for the purposes of deception, like lonelygirl15.
According to the definition submitted by a Wikipedia editor, "the term—a reference to the manipulation of a simple hand puppet made from a sock—originally referred to a false identity assumed by a member of an Internet community who spoke to, or about, himself while pretending to be another person.
The term morphed to include other misleading uses of online identities, such as those created to praise, defend or support a third party or organization, or to circumvent a suspension or ban from a website. A significant difference between the use of a pseudonym and the creation of a sockpuppet is that the sockpuppet poses as an independent third-party unaffiliated with the puppeteer. Many online communities have a policy of blocking sockpuppets.