An addictive good is one whose consumption eventually and reliably reduces the marginal utility of all other goods*.

It is because of its effects on the perceived utility of all other goods that an addicitive good is distinct from one that is moreish. A good is moreish if it takes willpower to resist consuming more after having eaten a little e.g. a chocolate digestive might be moreish.

A good is addictive if it reduces motivation to do anything relative to motivation to consume the addictive good.

*An important exception is a good that is a complement for the addictive good. For example, heroin is an addictive good because it reduces the marginal utility of books, cars, roast beef, going to the pub etc. but because syringes are a complement for heroin, heroin addiction increases the marginal subjective utility of syringes.