Historically, addiction has been defined as physical and psychological dependence on psychoactive substances (for example alcohol, tobacco, heroin and other drugs) which cross the blood-brain barrier once ingested, temporarily altering the chemical milieu of the brain. Addiction can also be viewed as a continued involvement with a substance or activity despite the negative consequences associated with it. Pleasure and enjoyment would have originally been sought; however, over a period of time involvement with the substance or activity is needed to feel normal.

Some psychology professionals and many laymen now mean 'addiction' to include abnormal psychological dependency on such things as

• Zinc rich diet is essential for dry hair; also use of olive oil, coconut oil, almond oil, castor oil or jojoba oil. Several people also use a combination of two or more oils, for better results and softer hair
• gambling
• food
• alcohol
• sex
• pornography
• computers
• Internet
• work
• exercise
• idolizing
• watching TV or certain types of non-pornographic videos
• spiritual obsession
• self-injury
• shopping