Refers to an entity outside of the buyer and seller arrangement. The buyer is considered the first party (similar to “I” being a first person pronoun), the seller the second party, and anyone supplying things from outside that sphere of influence is considered third party. It is a commonly used term in the high tech world we live in today because many working systems involve third party add-on items. For example, if you add RAM to a computer that comes from some other vendor it is considered third party. Sounds you add to your keyboard that come from some other maker are considered third party sounds. A ProCo cable added to a studio setup consisting of many different kinds of equipment is not normally considered third party. An Apogee A/D converter card installed into your Yamaha recorder is considered third party. A Yamaha converter card would not be third party…unless the card is made by someone other than Yamaha (a third party), which is a fine distinction. As often occurs with jargon, the phrase isn’t used with a great deal of precision in terms of exactly what it means.