My two cents:
Product design is the combination of interaction and interface design.
By interaction, I mean wireframes, user flows, product conceptualisation. This must be done at the first stage of product design.
Interaction also includes the visual and non-visual elements that will be presented to the user to communicate with the product (like the existence and positioning of buttons, animations and sounds).
By interface, I mean colours, typography, the overall tone, the identity (logo and co.) as well as copywriting and branding.
Interaction is how it works, interface is how it looks.
Interaction is usability (how usable the product is) and interface is aesthetics (how beautiful, delightful, etc.) the product is.
Discussion welcome: @usabbag