An action noun is a deverbal noun that refers to the event or action itself, i.e. not to a participant of the event.
English replacement is an action noun derived from the verb replace.
Words like eruption and downfall would generally also be placed in this category, although erupt and fall down are not actions, but non-agentive events. So strictly speaking the term event noun would be more appropriate. However, event is used in variety of senses, so event noun is not ideal either.
The Latin form of the word (nomen actionis) was widely used in the 19th and early 20th century. Action noun is simply a translation of this.