(Article is taken from Yu-gi-oh wiki)
"Nomi" （「のみ」） is a special term that refers to some monsters. If a monster is a "nomi monster", then it means that the monster cannot be Normal Summoned or Set. It can neither be Special Summoned except by fulfilling a special requirement. Examples of Nomi monsters include "", "", and "". 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 also a type of monster called "Semi-Nomi".