A status gained through the skills Stealth and Hidden Resurrect, and also the item Smoke Bombs.

Being Stealthed blocks regular attacks such as:

Shuriken, Phosphor Powders, Caltrops and also the skills: 

Steal, Cold Steal, and Heal.

As soon as you execute your first attack, you lose your Stealthed status. Your first attack while Stealthed will not show up in the victim's Status. Certain attacks and skills that DO NOT remove the Stealthed status upon execution are:

Dim Mak Scroll, Amanita Mushroom, Smoke Bombs, Ginseng Root, Tiger Salve, Sight, Steal, and Poison Touch (skill belonging only to Viper Ninjas.)