The Outer Worlds was a term referring to a group of planets at least partly under Federation control, as distinct from the Inner Worlds.
The degree to which the terms Outer Worlds and Outer Planets can be used interchangably is obscure.
It seems likely that the Outer Worlds were less strictly controlled by the Federation and more subject to insurrectionist activity: Blake referred to an existing revolt in the region prior to his attack on Federation computer control.
The styling of the President of the Federation included the phrase Lord of the Inner and Outer Worlds.