Feels like Gatsby. Like...how many parties can you handle in one night?
You're getting married. There are a lot of third parties involved (not that kind of third party...that doesn't come into play until a few years later, when things start to get stale). You've got the person performing the ceremony. You've got everyone's parents, who have an opinion on everything from the guest list to the type of flowers. You've got the caterer, the band, the wedding parties, the limo driver, that little girl who's going to dance with the bride later and become the star of the reception...lots of people with an interest in the wedding, who aren't actually the main parties getting married.
That distinction defines a third party. It's not the main principals of a deal. However, they're involved in the process, or have an interest in the outcome.
You buy a house from someone. You and the seller are the principals. But lots of third parties get involved as well. The bank providing the mortgage is a third party. The company acting as escrow agent. The realtors. Your parents again, who still have opinions about the flowers.
