Psychological safety was defined by Prof Amy Edmondson as ‘the shared belief that one will not be punished or humiliated for speaking up with ideas, questions, concerns, or mistakes, and the team is safe for interpersonal risk taking.’
For a deeper dive, you can read her book The Fearless Organization: https://fearlessorganization.com/the-fearless-organization.
You can also see Amy speaking at TED: https://www.youtube.com/watch?v=LhoLuui9gX8