According to the American Psychiatric Association, it's a widespread and persistent pattern of social inhibition, feelings of inadequacy, and fear of negative evaluation which starts by early adulthood, and that interferes with a person's functioning and/or causes significant distress. Also, at least four of the following must be present:

• Avoids occupational activities that involve significant interpersonal contact, because of fears of criticism, disapproval, or rejection
• Is unwilling to get involved with people unless certain of being liked
• Shows restraint within intimate relationships because of the fear of being shamed or ridiculed
• Is preoccupied with being criticized or rejected in social situations
• Is inhibited in new interpersonal situations because of feelings of inadequacy
• Views self as socially inept, personally unappealing, or inferior to others
• Is unusually reluctant to take personal risk or to engage in any new activities because they may prove embarrassing