Nomi

"Nomi" is a special term that refers to some monsters. If a monster is a "nomi monster" then it means that that monster cannot be special summoned, except by its own effect. Examples of Nomi monsters are Armed Dragon LV10, Doom Dozer, and Raviel, Lord of Phantasms. You can identify a Nomi monster because it will have "This card cannot be Normal Summoned or Set. This card cannot be Special Summoned except by..." in its text. There is a List of Nomi Monsters.

There is also a type of monsters called Semi-Nomi.