The constant use and reliance on something or a substance (drugs, alcohol, gambling, etc.)
Typically, addiction begins as a means to feel better due to stressors in life but can quickly turn into a reward and dopamine rushed system resulting in problems and leading to long-term consequences.  
Addiction usually results in one's life spinning further out of control and can negatively impact relationships and overall health. Psychotherapy and rehabilitation can help improve their well-being and decrease their reliance.