An epanorthosis is a figure of speech

Figure of speech

A figure of speech is the use of a word or words diverging from its usual meaning. It can also be a special repetition, arrangement or omission of words with literal meaning, or a phrase with a specialized meaning not based on the literal meaning of the words in it, as in idiom, metaphor, simile,...

 that signifies emphatic word

Word

In language, a word is the smallest free form that may be uttered in isolation with semantic or pragmatic content . This contrasts with a morpheme, which is the smallest unit of meaning but will not necessarily stand on its own...

 replacement. The example "thousands, no, millions!" is a stock example. More often, however, epanorthosis signifies immediate and emphatic self-correction, and as such often follows a Freudian slip

Freudian slip

A Freudian slip, also called parapraxis, is an error in speech, memory, or physical action that is interpreted as occurring due to the interference of some unconscious , subdued, wish, conflict, or train of thought...

 (either accidental or deliberate).

Examples:

• "The psychologist known as Sigmund Fraud—Freud, I mean!"
• "I've been doing this for six weeks!—er, days, that is."
• "Man has parted company with his trusty friend the horse and has sailed into the azure with the eagles, eagles being represented by the infernal combustion engine—er er, internal combustion engine. [loud laughter] Internal combustion engine! Engine!" – Winston Churchill

Here the words in italics are technically the epanorthoses, although all of the words following the dash may be considered part of the epanorthosis as well. Striking through words is another way of demonstrating the effect.