A phony name made up by a user in order to masquerade as someone else on the Internet. Sock puppets can make controversial comments or vote for or against a cause without revealing their identity. They may respond to their own X/Twitter or blog posts praising the articles they wrote themselves or disagree with comments criticizing them on other sites.
The term comes from a pretend person made by placing a sock over one's hand. See
meat puppet and
astroturfing.