Armor class

In Dungeons and Dragons, your armor class is a number representing your defense against attacks from monsters. NetHack borrows this concept. Because you always know your AC, you can identify the enchantment of armor by wearing it. For example, +1 armor lowers your AC by one more than normal.

A character with no armor and protection has AC 10. It is best to reduce your AC below 0. An ascension kit usually includes an AC below -20, -30, and sometimes -40.

Having a good AC is not enough to protect from some attacks; you also need to obtain resistance. In particular, reflection is a good idea.