Wifi roaming terminology I'd like to know if the distribution system allowing roaming between access points is considered as being a part of the ESS (extended service set).

To directly answer your question, I will provide a quote directly from the 802.11-2012 standard:

> An ESS is the union of the infrastructure BSSs with the same SSID connected by a DS. The ESS does not include the DS.

This makes it very clear that while the DS is essential to the establishment of the ESS, the DS is not part of the ESS.