此后 (cǐ hòu) is a formal, written word meaning 'after this, thereafter', often used at the start of a narrative to refer to a point in time just mentioned. It is more formal than the general 以后 (yǐ hòu) and, unlike 后来 (hòu lái), can refer to future events as well as past ones. 此后 does not take a specific time phrase (e.g., 'three days later' uses 之后 instead).