What is the name of the text that surrounds markup in mixed content?

["Costello, Roger L." <costello@mitre.org>]

Hi Folks,

This <Letter> element has mixed content:

<Letter>
    This is to inform you that it is
    <em>urgent</em>
    you contact us regarding your eligibility.
</Letter>

The <Letter> element has a child <em> element, which has text surrounding it.

Is there an official name for the text that surrounds the child element(s) in mixed content? Is it just called the "surrounding text"? 

/Roger