Synthetic people built on research into Data, outlawed after an attack on Mars and later recognised by the Federation as living beings.
Synths are artificial beings, from simple android workers to fully sentient people who are often indistinguishable from humanoids. The most advanced were created from research into Data's positronic mind, and some, like Soji Asha and her twin Dahj, were designed to believe they were human.
In the late 24th century, synthetic workers at the Utopia Planitia shipyards on Mars attacked, destroying the yards and setting the planet ablaze. The Federation responded by banning synthetic life, and scientists working in the field went into hiding. A group of synths was secretly raised on the planet Coppelius.
In the first season of Picard, Jean-Luc Picard sought out Soji and travelled to Coppelius as the Romulan Zhat Vash, who feared synthetic life, sent a fleet to destroy it. Soji chose not to summon a powerful synthetic alliance against organic life, and after the crisis the Federation lifted its ban on synthetic life.