third party

A third party is an entity that is involved in some way in an interaction that is primarily between two other entities. 

A contract might be, for example, between a software company that creates a mobile app and an end user. From the company's perspective, it is the first party and the end user is the second party; from the end user's perspective those positions are switched. The third party in this example might be a marketing firm or an ad network. 

The third party may or may not be officially a part of the transaction between the two primary entities and may or may not be interacting transparently and/or legally. Here are a few more examples of contexts in which third parties are mentioned:

• Eavesdropping is the covert interception of a private communication between two individuals by a third party.
• In a man-in-the-middle attack, a third party intercepts communications between two parties, usually an end user and a website, typically to access user data.

• A third-party cookie is placed on a user’s computer by a website from a domain other than the one the user is visiting.

• A 3PL (third-party logistics) is a provider of outsourced logistics services.
• A managed IT service is an information technology (IT) task provided by a third-party contractor. 
• An open relay (sometimes called an insecure relay or a third-party relay) is an SMTP email server that processes email messages that are neither for nor from any local user.