Addiction is the constant need to use certain substances or engage in particular behaviours.
It isn’t limited to substance use (drugs and alcohol); addiction can also show up as impulse control (gambling, self-harming, kleptomania, pyromania) and behavioural (internet, pornography, sex, video games, working, exercising, shopping, food).
An activity or substance becomes an addiction when you use it to avoid or deal with activities, emotions or thoughts. Once you stop or slow down, the withdrawal symptoms begin.