Definition: Internet Transit is the business relationship whereby one ISP provides (usually sells) access to all destinations in its routing table .
Figure - Transit Relationship - selling access to the entire Internet.
Internet Transit can be thought of as a pipe in the wall that says "Internet this way".
In the illustration above, the Cyan ISP purchases transit from the Orange Transit Provider (sometimes called the "Upstream ISP"), who announces to the Cyan ISP reachability to the entire Internet (shown as many colored networks to the right of the Transit Providers). At the same time, the Transit Provider propagates the Cyan routes across the Internet so that all attached networks know to send packets to the Cyan ISP through the Orange Transit Provider. In this way, all Internet attachments know how to reach the Cyan ISP, and the Cyan ISP knows how to get to all the Internet routes.
Some Characteristics of Internet Transit
Transit is a simple service from the customer perspective. All one needs to do is pay for the Internet Transit service and all traffic sent to the upstream ISP is delivered to the Internet. The transit provider charges on a metered basis, measured on a per-Megabit-per-second basis using the 95th percentile measurement method.
Transit provides a Customer-Supplier Relationship. Some content providers shared with the author that they prefer a transit service (paying customer) relationship with ISPs for business reasons as well. They argue that they will get better service with a paid relationship than with any free or bartered relationship. They believe that the threat of lost revenue is greater than the threat of terminating a peering arrangement if performance of the interconnection is inadequate.
Transit may have SLAs. Service Level Agreement and rigorous contracts with financial penalties for failure to meet service levels may feel comforting but are widely dismissed by the ISPs that we spoke with as merely insurance policies. The ISPs said that it is common practice to simply price the service higher with SLAs, with increased pricing proportional to the liklihood of their failure to meet these requirements. Then the customer has to notice, file for the SLA credits, and check to see that they are indeed applied. There are many ways for SLAs to simply increase margins for the ISPs without needing to improve the service.
Transit Commits and Discounts. Upstream ISPs often provide volume discounts based on negotiated commit levels. Thus, if you commit to 10Gigabits-per-second of transit per month, you will likely get a better unit price than if you commit to only 1gigabit-per-second of transit per month. However, you are on the hook for (at least) the commit level worth of transit regardless of how much traffic you send.
Transit is a commodity. There is debate within the community on the differences between transit from a low cost provider and the Internet transit service delivered from a higher priced provider. The higher price providers argure that they have better quality equipment (routers vs. switches)
Transit is a metered Service. The more you send or receive, the more you pay. (There are other models such as all-you-can-eat, flat monthly rate plans, unmetered with bandwidth caps, etc. but at the scale of networking where Internet Peering makes sense the Internet Transit services are metered.)