effectively impossible; practically impossible
Used when something is theoretically possible but so difficult or unlikely that it is treated as impossible in practice.
Carrying out this plan now is effectively impossible.
Under such conditions, success is practically impossible.
不可能 means 'impossible' in an absolute sense, while 事実上不可能 adds the nuance of being impossible for all practical purposes.
Compound of 事実上 (じじつじょう, 'in fact, practically') and 不可能 (ふかのう, 'impossible').