Lag may refer to any of the following:
1. In general, lag is the time difference between a performed task and its result. For example, the fraction-of-a-second pause between when a keyboard key is pressed and when it appears.
2. In gaming, lag refers to any noticeable slow response in an online game due to high latency or a poor connection. For example, on a slow Internet connection, your character or the environment they're in may stutter.
If lag only happens temporarily, it's called a lag spike.