Short for "Input/Output' and referring to 1) an in-line console module that contains controls for the input section, output section and monitor section. 2) A module in electronic gear containing input and output amplifiers for the device. 3) A digital port (connector) able to both receive digital data and output digital data. 4) A card or separate unit that converts audio to digital audio for input into a digital system and also do the reverse for the output.