inside, within (space)
räumlich — innerhalb eines Raumes
Spatial — within a space
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Innerhalb der EU kann man frei reisen.— Within the EU you can travel freely.
With bare numerals or article-less plurals, Genitiv sounds awkward — speakers insert "von + Dat": "innerhalb von drei Tagen", "innerhalb von Minuten". Full Gen ("innerhalb einer Woche") works fine.