Title Attribute

You can use the title attribute for definitions and footnotes which will appear when the user hovers over the text.:

Some Text

Footnote [1]

Click the HTML button and insert something like:

<span style="border-bottom:thin dashed green" title=" Definition for Some Text ">Some Text</span>
or
<span style="color:red" title=" Footnote text ">[1]</span>

For help with CSS style attributes see CSS How To

Note: Not all browsers will display the title attribute and not all screen readers will speak the text.