# What is Impression?

An impression is registered each time a notification appears on your subscribers' devices, representing a single instance of engagement. It's important to understand that achieving a 100% impression rate is not always feasible due to various factors:

1. **Uninstalled Browsers**: If a subscriber uninstalls the browser through which they receive notifications, impressions cannot occur.
2. **Closed Browser (MacOS)**: When a subscriber closes their browser, notifications cannot be displayed, impacting impression rates.
3. **Expired Notifications (Flash Sale Campaign)**: Notifications tied to time-sensitive events, such as flash sale campaigns, may expire before subscribers have the chance to see them, affecting impression counts.
4. **Disabled Background App Activity on Mobiles**: Some mobile users may disable background app activity, preventing notifications from being received and resulting in fewer impressions.
5. **Erased Site Data**: If a subscriber clears their browser's site data, they are no longer a subscriber.

Understanding these factors can help manage expectations regarding impression rates and optimize notification strategies for better engagement.