"Nomi" (のみ nomi, a Japanese adverbial particle that means "only") is a special term that refers to some Monster Cards. Nomi monsters cannot be Normal Summoned, Set or Special Summoned, except by fulfilling a special requirement. They always have the effect "Must be Special Summoned by (method), and cannot be Special Summoned by other ways" (formerly "This card cannot be Special Summoned except by (method)"), as well as the effect "Cannot be Normal Summoned/Set", if the monster in question is an Effect Monster.
Examples of Nomi monsters include "Armed Dragon LV10", "Doom Dozer", "Judgment Dragon", "Raviel, Lord of Phantasms", "Rainbow Dark Dragon", "Red-Eyes Darkness Dragon", "T.G. Halberd Cannon", "Star Eater", "Shooting Quasar Dragon", most "Infernoid" monsters, all "Nekroz" Ritual Monsters, all Masked HERO monsters, and most "Elemental HERO" Fusion Monsters. It is also to be noted that the Special Summoning of these cards can only be done while the card in question is in the owner's hand unless stated otherwise in card text.
There are also "Special Summon-only monsters", often called "Semi-Nomi monsters", with similar but somewhat less restrictive Summoning restrictions/conditions, like "Black Luster Soldier - Envoy of the Beginning"