What is the name of this Utility on a web page?

What is the name of this feature or utility in a web page?

On some sites, only a paragraph of a larger article shows. At the end of the paragraph there is a + sign. If the user clicks the + sign, the entire article expands, making the actual page longer as well as the page accomodates the extra text.

I am sure the entire paragraph loaded with the web page as the extra text immediately shows when the + is clicked.

If the + is clicked again, the article shrinks back to the first paragraph, like it was viewed in the beginning.

Do you know the name of this function?

Collapsible Text I think.

Expandable/Collapsible Content

^ that way works great, it can also be done with ajax (http request) to pull in the data from a separate file without reloading the page.