The fictional planet in Ursula K. Le Guin's The Left Hand of Darkness is Gethen.
Gethen is a remote world in Le Guin's Hainish universe, and its inhabitants also call it Winter because of its severe, ice-covered climate. The novel follows Genly Ai, an envoy from Terra, as he tries to persuade Gethen's nations to join the Ekumen, a loose alliance of human worlds.
The planet's society is shaped by the biology of its people, who are ambisexual rather than permanently male or female. For most of the lunar cycle they are sexually latent; during kemmer, they temporarily develop sexual characteristics. This premise allows Le Guin to examine gender, politics, intimacy, and assumptions about human identity.
Earth is called Terra in the novel, while Hain is the ancient world associated with the wider Hainish setting. Winter is a nickname or descriptive name for Gethen, not the planet's primary name. These related terms explain why Gethen is the single canonical answer to the question.