Unbundled access is an often practiced form of regulation during liberalization, where new entrants of the market (challengers) are offered access to facilities of the incumbent that are hard to duplicate (e.g. for technical or business case reasons). Its applications are mostly found in network-oriented industries (like telecommunication, mail and energy) and often concerns the last mile.
Unbundled access is similar to Bit-stream access, where the incumbent provider gives competitive access not to the actual copper wire of the local loop, but to a high-speed ADSL data connection. Both setups ensure competition for the backhaul but leave "last mile" infrastructure the responsibility of the incumbent carrier.