An activation is a small rectangular box on a lifeline. This activation box represents the point when an activity was active. The top of the box represents the start of the activity and the end of the box represents the end of the activity.
Let's see how it looks in a diagram:
![](http://images-20200215.ebookreading.net/7/1/1/9781788296229/9781788296229__hands-on-object-oriented-programming__9781788296229__assets__b7dcc16c-3e5f-4a20-8458-d44fb43d3413.png)