From a scientific viewpoint, boredom is very poorly defined. Some research groups asserting that boredom is associated with very low levels of psychological 'arousal' and other asserting the opposite (i.e. boredom can induce intense agitation).

It currently falls into the category of an emotionplugin-autotooltip__plain plugin-autotooltip_bigEmotions

Theories about emotions stretch back at least as far as the Stoics of ancient Greece, but to date, no definitive taxonomy of emotions exists - though several have been proposed. And many of the explanations about the apparent biological necessity for some of the emotions (e.g. 'boredom' or 'shyness or 'embarrassment') remain sketchy and controversial. , and although the causes are obvious - lack of opportunity for interesting activity - the reasons why it has such profound effects are not known.

Boredom effects can be so intense as to cause serious physical symptoms such as headaches, dizziness, digestive problems, and even, in severe cases, cardiac problems.

It can also cause psychological problems such as depression and apathy. As well as leading to sociological syndromes such as workplace 'Boreout'

Given its powerful effects, it has been studied quite sparsely. For an example recent publication see : Characterizing the psychophysiological signature of boredom, Experimental Brain Research 232(2)

Note: Distress caused by boredom is by no means restricted to humans, as anyone who has visited a zoo, or seen caged birds etc will know.