An ellipsis is punctuation that is used to show where words have been left out. The ellipsis is usually formed by three periods (four if the ellipsis comes at the end of a sentence).
The word ellipsis comes to English via a Greek word, elleipein, meaning to fall short or leave out. The plural of the noun is ellipses. In the sentence, "'What the . . . !' she exclaimed.", the ellipsis replaces an expletive. The severity of the expletive is left to the reader's imagination.