How I define “interface design”
I use the words “interface design”, “interaction design”, and “visual design” often. Here are the definitions I personally use.
You design interactions when you make decisions about how an interface responds to input.
The “input” is often human input, but does not need to be. The interface could respond to the passing of time as an input, or to when the software is turned on for the first time.
These responses include specialties, like animation design and sound design.
Visual design is the creation or curation of everything a person sees in an interface.
I say “creation or curation” because some elements in an interface, like a button, are created from scratch. And some, like a photograph, are chosen.
Interface design is the combination of interaction and visual design. Importantly, there is a lot of overlap between the two. Most of visual design exists for the benefit of interaction design. And the most common method of response to input is visual.
Interface design is informed by other relevant fields. Research, usability testing, information architecture, etc.
An “interface” is any way for a person to use software. This includes mostly-visual interfaces, but also other types like voice-only interfaces.