The Mongol assembly that proclaimed Temüjin as Genghis Khan in 1206 was the kurultai.
A kurultai was a formal gathering of Mongol and steppe elites. Chiefs and military leaders used such assemblies to recognize rulers, settle political questions, assign commands, and coordinate campaigns. In 1206, after Temüjin had defeated or allied with major rival tribes, the assembly at the Onon River recognized him as leader of the united Mongol nation and bestowed the title Genghis Khan.
The kurultai was not simply a parliament in the modern sense. Its authority depended on the participation of powerful aristocratic and military figures, and attendance could affect whether a decision was accepted as legitimate. Later Great Khans were also formally elected or confirmed through kurultai gatherings.
Common mix-ups include the yassa, a body of laws associated with Genghis Khan; the keshig, his elite imperial guard; and the yam, the relay-post communication system. None of those was the assembly that elevated Temüjin in 1206.