Rollover Text on Text

My request is simply to learn how to make a single word in a bunch of text, have a rollover feature that would display a small text block with a definition for that word.

ie: in the following sentence, the word “Nixon” when hovered over with a cursor, would reveal a box of text containing: “37th President of the USA”

It was in his final year that Nixon reached the most controversy.

Many thanks to you all…