"350 ppm" refers to a measurement of carbon dioxide (CO2) concentration in the atmosphere, specifically 350 parts per million. This means that for every one million air molecules, 350 of them are CO2. This level is significant because it is considered a safe threshold for maintaining a stable climate and avoiding severe impacts of climate change. The term gained prominence through the work of climate scientists and organizations like 350.org, which advocate for reducing CO2 levels to this benchmark. Exceeding 350 ppm is linked to global warming and environmental issues, prompting efforts to lower emissions and promote sustainability.