When your Pokemon's Speacial Defense raise it mean it is stronger to it's power. Like when your Kadabra's Speacial Defense raise it will be stronger Psy moves.
Edit from lipantwort:
Special defence is a stat which decreases loss of hitpoints from special attacks. Almost every attack in Pokemon is either categoried as special or physical. When physical attacks are used, defence is one of the factors which determines how big the HP loss is. When special attacks are used, the special defence is a factor which determines how big the HP loss is.
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Special defense is resistant to special attacks Like flamethrower is a special attack And your pokemon has a high special defense then it should resist it.
it depends because special defense is defense against special attacks like secret power but defense defends against any moves apart from special type moves(they are quite rare).
Any water, ice, or grass type Pokemon. :) I recommend using special attacks, since Ground and Rock type Pokemon usually have good defense and low special defense.
Here is the best of the best when it comes to defense give it up for Shuckle! He has amazing defense and special defense making him a tough Pokemon to defeat but that's why everything else but HP is terrible!
If you use medicine it will level it up like calcium or what you want. you can also catch legendary Pokemon and they have very high defense and hp.