Concealment

From NWNWiki

Concealment is a chance that your opponent will miss you when he would normally hit otherwise. For example, someone with 50% concealment (from something such as improved invisibility) will be missed half the time (when they normally would be hit).

Concealment can be partially negated by the blind fighting feat, which lets you re-roll for a second chance when failing a concealment check. For example, if your first roll results in a miss due to concealment, you can roll again. So a 50% concealment would effectively become 25% against an opponent with blind fighting.

An epic character may take the self concealment feat to achieve a permanent 10% concealment bonus. This feat can be retaken up to five times for 50% permanent concealment.

Other than multiple self concealment feats, concealment does not stack. Only the highest concealment is used. The maximum concealment that a player character can have is 50%.

See invisibility and true seeing do not penetrate concealment.