Rendering engine
Also called: Browser engine
The part of a browser that turns HTML and CSS into what you see: Blink, WebKit or Gecko.
Definition
Chrome, Edge and Samsung Internet use Blink; Safari uses WebKit; Firefox uses Gecko. On iOS, browsers use WebKit because of Apple's rules (only in the EU may they use their own engine, and few do), so testing 'Chrome on iPhone' mostly tests Safari's engine.
In practice
A layout bug appears in Safari and in Chrome on iPhone, but not in Chrome on Android, so it's a WebKit issue.