The means by which you interact with a product is called the user interface.
The means by which you interact with a product – be it physical or digital – is called the user interface. So to take this website as an example, the UI consists of its layout, the navigation, the search box, the links… basically all the visual components that you can use to interact with the site.
Much of what we call UI nowadays is really GUI (graphical user interface), i.e. the images and icons on your computer or mobile.