credible threat

For a threat to be valid it must be credible.

If one makes a threat without any clear means or motivation for following through, then the threat is meaningless, and will likely be ignored.

I have an english teacher who always writes "Reading Quiz (?)" on the agenda each day. To make the threat of a reading quiz credible, he gave one all but a couple days at the beginning of the year.

In game theory a credible threat is also known as a sub-game perfect Nash equilibrium or SPE. SPE's tend to be credible because they represent a strategy set which is an absolute best response.