The term middle mile arose to distinguish this part of the network from the last mile, which means the local links which provide service to the retail customer or end user, such as the local telephone lines from the telephone exchange or the coaxial cables from which connect to the customer's equipment.
Internet bandwidth is relatively inexpensive to purchase in bulk at the major Internet peering points, and access to end-customer ports in the incumbent operator's local distribution plant (typically where local loop unbundling is mandated by a telecom regulator) are also relatively inexpensive relative to typical broadband subscription costs.
For this reason, many proposals for government broadband stimulus initiatives are directed at building out the middle mile.
Two examples are the Network New Hampshire Now and Maine Fiber Company in the Northeast US, both funded largely by the National Broadband Plan (United States) to connect all community anchor institutions.
This sometimes leads to controversies, such as the NRECA opposition to pole attachment tariff changes [1] motivated by the US plan.