Nightmare (difficulty)

From Diablo Wiki

Nightmare is the difficulty level following Normal. Although you will have the creatures with the same appearance and the same quests as in Normal, they will be much harder and a few might use more spells. Furthermore, while in Normal difficulty dying loses you gold, in Nightmare you will also lose experience.

For having access to the Nightmare difficulty you must meet one of the following requirements:

• If you are playing an expansion character, you must have defeated Baal at Normal difficulty;
• If you don't, you must have defeated Diablo at Normal difficulty.

Difficulties
Normal • Nightmare • Hell