A webview is a webpage embedded in a mobile app. A session involving webviews will use a mix of our web SDK and our mobile SDK. As a result, for any given session, two session IDs will be created (one for the web portion and one for the native portion).
Webviews can be distinguished from true website visits by looking at the Browser property of a session. For example, webviews on iOS will have a browser value such as Mobile Safari UIWebView.