It's a film-forming plasticizer. It helps make nail polish flexible and durable, and also helps improve how well the polish sticks to the nail.. Normally banned from cosmetics in Europe, It is composed of toluene. And toluene has been shown to be a potent neurotoxin that can cause neurological damage, impaired breathing, hearing loss, reduced brain function, etc.
LAG has replaced it with sorbitol, a bio-sourced plasticiser (potato)