Definition: Internet Peering is the business relationship whereby companies reciprocally provide access to each others’ customers.
To illustrate peering, considerthe figure below showing a much simplified Internet; an Internet with only three ISPs: WestNet, MidNet, and EastNet.
WestNet is an ISP with green customers, MidNet is an ISP with blue customers, and EastNet is an ISP with red customers.
WestNet is in a Peering relationship with MidNet whereby WestNet learns how to reach MidNet's blue customers, and MidNet reciprocally learns how to reach WestNet's green customers.
EastNet is in a Peering relationship with MidNet whereby EastNet learns how to reach MidNet's blue customers, and MidNet reciprocally learns how to reach EastNet's red customers.
Peering is typically a free arrangement, with each side deriving about the same value from the reciprocally arrangement. If there is not equal value, sometime one party of the other pushes for a Paid Peering relationship.
Important as well is that peering is not a transitive relationship. WestNet peering with MidNet and EastNet peering with MidNet does not mean EastNet customers can reach WestNet customers. WestNet only knows how to get to blue and green customers, and EastNet knows how to reach only blue and red customers. The fact that they both peer with MidNet is inconsequential; peering is a non-transitive relationship.