Music Production
Audio interface
The box between your mic, instrument, computer, and monitors that converts audio between analog and digital.
The device that converts microphone and instrument signals into digital audio your computer can record, and converts playback back out to your monitors and headphones — usually with lower latency and better sound quality than a computer's built-in audio. The main things that separate one from another are how many simultaneous inputs and outputs it has, the quality of its mic preamps, and how it connects (commonly USB, with Thunderbolt on higher-end units).
Spec example
a typical home-studio interface offers 2 inputs, while band or multi-mic setups may need 4, 8 or more.
What it means when you're buying
count the maximum number of sources you'll record at once and buy enough inputs for that, since you can't easily add more later without a second interface.