Perfectionism is defined as a meticulous pursuit to reach excellence.
In psychology, it is an ego self-defense mechanism: it is a form of procrastination when it is used to postpone tasks ("I can't do anything on this until I know the perfect thing to do so that I won't make any mistakes"), and a form of self-deprecation when it is used to excuse poor performance or to seek sympathy and affirmation from other people ("I must be perfect, but I keep falling short of my own goals, why am I so terrible?").
See also