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Lowers the target's evasion by one stage. It will also remove Reflect, Light Screen, Safeguard, Mist, Spikes, Stealth Rock, and Toxic Spikes set up on the target's field. Pokemon protected by Magic Coat or the ability Magic Bounce are unaffected and instead use this move themselves. In double or triple battles, it will also remove the fog weather condition while in battle. This move cannot be blocked by Substitute, but Substitute does prevent evasion from being lowered. Has an in-game effect that clears deep fog from an area.
Defog is famous as the least imaginative HM ever, by popular opinion. It loses its HM status in BW, much to the relief of players everywhere. It works similarly to Rapid Spin, removing several field effects. However, its application in singles is undesirable, since if used, it will remove your own entry hazards from your opponent's side. For this reason, it is not simply useless, it is actually totally counterproductive to almost anything you could choose to do. If you want to remove screens, Brick Break is much better. Unless you have ESP and your opponent is using a Magic Bounce Espeon, do not use this move.
In theory, it could be used in double and triple battles, where it's possible to target your own Pokemon, but entry hazards aren't used much in that competitive environment, and users of Taunt are very common, so that Rapid Spin is almost always better.