Design.
A design solves a problem. Construction of any kind is possible in a vacuum – just put things together with screws, crazy glue, method calls or file includes. But design anchors construction - it guides the building of a tool to solve an actual problem.

Interfaces.
What is an interface? Broadly, an interface is the locus of an exchange - a conversation. So where’s the problem? The problem to design for is the conversation itself. So interface design is the guided construction of tools that help a conversation, any conversation. This conversation can involve other humans, computers, phones, street signs, doors, hammers, you name it.

Usability.
Usability is a measurement of how well the tool, through its design, facilitates the conversation. How usable is that web page/program/street sign/hammer? How easily can I have an exchange with it? If you replace an old tool with a new tool, and the conversational exchange gets easier, that’s a good sign. If it doesn’t, or it gets easier but suddenly other get harder, that’s a bad sign.