One more key element of boredom is control. Boredom often occurs when you have little control over your situation. Waiting rooms, lectures, and airline gates are all places where you have little control over your situation. Normally, we react to unpleasant situations by changing the situation. If you don’t like a book you are reading, for example, you close it and do something else. Boredom happens when you are unable to change the situation.  
This example leads to another key aspect of boredom. As Eastwood, Frischen, Fenske, and Smilek point out, bored people become aware of their difficulty concentrating. As a result, bored people often try to amuse themselves by daydreaming and letting their mind wander. Interestingly, while mind wandering helps people to keep their minds occupied, studies suggest that the more your mind wanders, the more bored you feel. The idea is that you recognize that this daydreaming is meant to occupy your mind, and so you realize that the situation is boring.
The French loanword ennui comes from the very same Late Latin word that gave us annoy — inodiare ("to make loathsome"). We borrowed ennui several centuries after absorbing annoy into the language. Ennui deals more with boredom than irritation - and a somewhat specific sort of boredom at that. It generally refers to the feeling of jadedness that can result from living a life of too much ease. The poet Charles Lloyd described it well in his 1823 Stanzas to Ennui when he referred to that world-weary sensation as a "soul-destroying fiend" which visits with its "pale unrest / The chambers of the human breast / Where too much happiness hath fixed its home."