An action noun, also called nomen actionis, is a deverbal noun that refers to an action or event. Deverbal simply means that the root of the word is a verb, but in the context of a sentence, it is used as some other part of speech. The definition of an action noun may like an oxymoron, since verbs, not nouns, are typically described as "action words." An action can also be a "thing," however, as in the definition of a noun as a "person, place, or thing." Action nouns may be verbals, other formations from verbs, or not derived from a particular verb at all.
Some action nouns are non-finite verbs, which means they cannot be the main verb of a sentence. A gerund, which is a verbal that acts as a noun and is formed by adding -ing in English, can often be used as an action noun. In the sentence "Towing a car is expensive," for instance, "towing" is a gerund that refers to the act of towing, which makes it an action noun. Although "tow" is usually a verb, in this instance it acts as the subject noun of the sentence. Other words such as "blessing," which are as common in their noun form as their verb form, are also gerunds.
An infinitive is another type of verbal that can be used as an action noun in some languages. In English, it is formed by "to" plus the base form of the word, as in "to indicate." Since this is a two-word phrase, it is not really considered an action noun. In many other languages, however, the infinitive is a single word and, therefore, an action noun, as in the Latin amare, meaning "to love."
Action nouns may also be formed from verbs in other ways, such as by adding -tion in English. For instance, "construction" is a deverbal noun since it comes from the verb "construct." It is an action noun, however, if it is used to refer to the act of constructing, as in "The construction of the building will be completed in February." If it is used as an adjective or to refer to something that has been constructed, it is not an action verb because it does not refer to the action itself. In the phrase "a construction site," the word "construction" is an adjective; in "A construction of duct tape and newspaper," "construction" refers to an object rather than an action.