As the letter Z is the commonly agreed upon abbreviation for impedance, then Hi-Z simply refers to “hi-impedance”. This refers to the input or output impedance of a device (in our cases an audio device). Precisely what Hi-Z means, and how it is applied in the audio industry, is not entirely concrete. In general devices with impedances up through 600 ohms are said to be “low impedance”, while devices with impedances of several thousand ohms and up are considered “high impedance”. Typically we only come in to contact with these generic terms on microphones (usually low cost microphones), some direct boxes, and certain types of line inputs (on mixing boards, some tape decks, etc.). A typical guitar, for example, generally needs to be connected to a Hi-Z input. Otherwise the electronics will be “loaded down” and the sound will be significantly altered. A Hi-Z microphone, which we don’t encounter very often in pro audio (we generally use low impedance mics), definitely needs to be connected to a high impedance input, and even then the cable length can’t be more than 10 or 20 feet before the signal degrades.