Special Summon-only monster

"Semi-Nomi" is a term that refers to a special sub-group of monsters. If a monster is a "Semi-Nomi Monster" then it means it must be special summoned by its listed effect before it can be special summoned by another effect. Aqua Spirit, Harpie Lady Sisters, and Black Luster Soldier - Envoy of the Beginning are all Semi-Nomi monsters. You can recognize a Semi-Nomi monster because it will have "This card cannot be Normal Summoned or Set. This card can only be Special Summoned by..." in its text (although some cards, like Black Luster Soldier - Envoy of the Beginning do not have the first sentence, that rule still applies). There is a List of Semi-Nomi monsters.

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