Chances are, you've seen the email client, platform, and environment reporting available in the Mailing Overview report. This article breaks down a couple of commonly reported environments called WebView.
Android WebView is a system component for the Android operating system (OS) that allows Android apps to display content from the web directly inside an application. Android is powered by Chrome.
Mobile Safari UIWebView
The UIWebView is different from the ordinary Safari browser, as it is not a stand-alone browser, but merely browser functionality that is embedded in a third party app that allows the app to display content from the web. iOS is powered by Safari.
Both are components of an operating system, powered by a browser, that allows display of web content inside an application on the device. WebView for any OS is not an email client, nor is it a stand-alone browser.