‌定义：设函数<img src="https://static.zxtiku.com/wangke/imgs/topic/60545f7a87e6d206cb7da6e5ea03314e.png"/> 在点<img src="https://static.zxtiku.com/wangke/imgs/topic/325b4176bc3d6ccaa84d88bf0ee5ac2b.png"/>的某左邻域内有定义，若极限<img src="https://static.zxtiku.com/wangke/imgs/topic/b4f89822fea88aaf66ccc77d55d95183.png"/>存在，则称该极限为函数<img src="https://static.zxtiku.com/wangke/imgs/topic/60545f7a87e6d206cb7da6e5ea03314e.png"/> 在点<img src="https://static.zxtiku.com/wangke/imgs/topic/325b4176bc3d6ccaa84d88bf0ee5ac2b.png"/>处的（），记作<img src="https://static.zxtiku.com/wangke/imgs/topic/54062cacec62e2ccb16a8ac7cdf4a865.png"/>‌