Single-command environment variables, to be precise. The same thing that gets created when you use "export", but isn't created when you use "=" on a line on its own:

  $ FOO=BAR env | grep FOO   # Exists as an environment variable passed to env
  FOO=BAR
  $ env | grep FOO           # But only for that command
  $ FOO=BAR                  # Creates a variable (not an environment variable)
  $ env | grep FOO           # As shown by not being passed down
  $ echo $FOO                # But still accessible in this context
  BAR
  $ export FOO=BAR           # Promotes it to an environment variable (the "=BAR" is optional since it's already defined)
  $ env | grep FOO           # And as such is now visibly passed down
  FOO=BAR

(Yes, bash does mix these together so it's easy to accidentally replace something you don't intend to)