上瘾 (shàng yǐn) means 'to become addicted (to something)' — it is a separable verb (离合词) that can be split, e.g. 上了瘾. It is the direct, neutral word for addiction to things like games or smoking. 沉迷 (chén mí) also means 'to be deeply absorbed', but it carries a stronger tone of warning or regret, and it cannot be split.