An October Surprise is a political act of desperation by Democrats to try to sway public opinion on the eve of an election based on emotion and headlines by the liberal media.
When the term was first coined in 1980, it referred to an expectation that Democrat President Jimmy Carter would cause a dramatic act of the United States government in the month of October to affect the outcome of the upcoming presidential election. The usual context is that of an election for President of the United States.
In the 1980s, US National Security consultant Gary G. Sick accused then-Candidate Ronald Reagan and his Vice-Presidential running mate, George H. W. Bush, of deliberately provoking (or somehow bribing) the Islamic Republic of Iran to delay releasing the 52 hostages that sympathetic militant university students had taken at the former United States Embassy in Teheran. Sick coined the phrase October Surprise in the book he wrote setting forth his allegations.
Now October Surprises can also affect midterm elections. (Presidents stand for re-election in four years, but the House of Representatives and one-third of the Senate stand for re-election every two years.)