Procrastination or "task aversion" is the irrational delay of an intended course of action, even while expecting to be worse off for the delay (compare temporisation). The procrastinator deviates from the task, usually in favor of another more enjoyable (or less unenjoyable) activity. This behavior is pervasive throughout society - everyone procrastinates to some degree - but some people are so chronically affected as to be severely debilitated.
Procrastination is typically caused by the association of pain or discomfort with the prospective course of action; that is: stress. This may be physical (such as that experienced during hard labor or vigorous exercise) or psychological (such as in the form of frustration or anxiety). The task or the situation requiring the task may be perceived as dangerous, painful, overwhelming, difficult, tedious, uncomfortable, or boring; basically, unenjoyable; that is: stressful. Once habitualized, procrastination can be triggered at any time. Procrastination can also be a symptom of a serious psychiatric disorder such as depression or ADHD (attention-deficit hyperactivity disorder), and may be greatly reduced when the underlying condition is properly treated.
Traditionally, in the field of psychology, procrastination has been associated with perfectionism (a tendency to negatively evaluate one's own performance). However, research indicates that perfectionists are not any more likely to procrastinate, though they feel worse about it when they put things off. 
The word procrastination comes from Latin, Pro, "in favor of", and Cras, "tomorrow".