A ɢᴇʀᴜɴᴅ is a type of verb, in particular an -ɪɴɢ verb that heads a non-finite verb clause when that entire clause is being used as a noun phrase, typically as the subject or object of a finite clause. Not to be confused with -ɪɴɢ words that are no longer verbs, like deverbal nouns or participial adjectives, a gerund accepts only verb modifiers and arguments, not those of nouns or adjectives.

Gerunds are verbs, not nouns

The English gerund is a type of non-finite verb ending in -ɪɴɢ. Modern grammars do not distinguish gerunds from participles, calling all -ɪɴɢ verbs gerund–participles.

In form gerunds are identical to present participles (which are also verbs, just like gerunds), deverbal nouns (ex-verbs that are now nouns), and participial adjectives (ex-verbs that are now adjectives).

The way you can tell which is which is by seeing how it’s used, not whether it ends in -ɪɴɢ, since they all do so. In all those cases, we’re dealing only with verbs, nouns, or adjectives. To be a gerund(–participle), it must be a verb not a noun or adjective.

The gerund heads gerund clauses, which are created by doing verb things to the gerund to create a multiword phrase. The resulting gerund clause can be used wherever a noun phrase is syntactically required, but a gerund itself is never a noun. The entire gerund clause can act as the subject or object of a verb, the object of a preposition, or as a noun adjunct.

Like any other verb, the gerund accepts adverbs, object complements, and (optional) subjects. It rejects articles, determiners, adjectives, or prepositional phase. If you find one that seems to take those, this is ɴᴏᴛ a gerund but a deverbal noun — an ex-verb, if you will.

Gerund example (a real verb)

Here is an example of an actual gerund inside a gerund clause. It is an actual gerund it is a verb not a noun, and occupies a clause being used as a noun phrase:

Loudly singing songs makes people happy.

The sentence subject is the entire gerund clause “loudly singing songs”. Because the word singing has both an adverb loudly and a direct object songs, it must be a verb.

Because it is acting as a noun phrase not an adjective phrase, it is a gerund not a participle (for those who make that distinction: modern grammars call all verbal -ɪɴɢ words gerund–particles).

Deverbal noun example (noun, not a verb so not a gerund)

In contrast, this is not a gerund because it is only a noun not a verb:

The loud singing of songs makes people happy.

The subject of the sentence is the noun phrase the loud singing of songs. You can tell that singing is a noun not a verb because it takes the adjective loud with the earlier article the, and it is connected to the adjectival prepositional phrase of songs,

Therefore this instance of singing is a noun not a verb. Since it is not a verb, it cannot be gerund.

Some grammars call these deverbal nouns, while others simply call them nouns. In a way, they are ex-verbs, nouns that began life as verbs but have since had their special powers as verb stripped from them.

Why do some people call the gerund a noun if it is not?

The point of confusion arises because a gerund clause doesn’t have to have any other words in it but the gerund alone. The overall clause acts as a noun phrase even though it has only a single word, so a one-word gerund clause can appear to be a noun if you don’t look too closely. But it is still a verb, as you will learn if you try to add words to it.

Non-technical dictionaries sometimes shorten all that up and refer to a gerund as a noun. as any linguist will tell you, this definition is technically incorrect.

As it often but not always does, the Oxford English Dictionary presents more nuance than that when it says that the gerund is:

A form of the Lat. vb. capable of being construed as a n., but retaining the regimen of the vb. Hence applied to forms functionally equivalent in other langs., e.g. to the Eng. verbal noun in -ing when used rather as a part of the vb. than as a n.

That is, with abbreviations expanded:

A form of the Latin verb capable of being construed as a noun, but retaining the regimen of the verb. Hence applied to forms functionally equivalent in other languages, for example to the English verbal noun in -ing when used rather as a part of the verb than as a noun.

The critical element here is the last part, “when used rather as a part of the verb than as a noun”.

For detailed linguistics matters, always consult linguistics texts, not non-specialist dictionaries.