The empty space or atmosphere that appears to provide no physical support; often used literally to describe something unsupported or appearing to float.
2.
USUKGlobalneutralidiomatic
Complete and inexplicable disappearance; to vanish without trace or apparent cause (as in 'vanish into thin air').
3.
USUKGlobalinformalfigurative
A metaphor for lacking substance, support, or resources; to exist or be expected to survive on insufficient or imaginary means (as in 'live on thin air').