In grammar, what do we call this specific usage of “this”?

Question

In spoken American English, often times people who are telling a story use the determiner "this" not as a demonstrative, but rather as a word that serves to emphasize the impact of something. Examples:

"I go inside and I see this beautiful girl..."

"We were in the woods and then this huge bear appears..."

"When we woke up, there was this huge fire."

In these examples, the narrator does not use "this" to refer to a familiar object. It doesn't mean that we were in the woods and "this bear right here, not that one, but this one I'm pointing to, appeared." "This" is somehow used for dramatic effect for something new and usually impressive.

The reason I am trying to determine exactly what kind of usage this is is because I am trying to resolve a translation problem that often occurs within my team and speakers of my native language. Most people just go ahead and translate it conventionally as a demonstrative, but the sentence doesn't make much sense in my native language, because the determiner "this" implies that the object is something known.

Has this been defined anywhere in grammar terms? Googling this has turned out to be a nightmare.

Answer 1

From Merriam-Webster's definition of the adjective this:

1 d : being one not previously mentioned —used especially in narrative to give a sense of immediacy or vividness
// then this guy runs in
// had this urge to go shopping

Answer 2

From Collins

This - 4. (determiner:)

In spoken English, people use this to introduce a person or thing into a story.

• I came here by chance and was just watching what was going on, when this girl attacked me.
• So I just walked up the steps into this big, beautiful church.