A circle is a self-governing and semi-autonomous team of equivalent people who collaborate to account for a domain.

A circle:

may be permanent or temporary
may be self-organizing
is accountable for its own development and its body of agreements
semi-autonomous:
A circle’s members act within the constraints of their domain.
Each circle can create value autonomously.
self-governing:
A circle’s members continuously decide together what to do to account for their domain, and set constraints on how and when things will be done.
equivalence of circle members:
All members of a circle are equally accountable for governance of the circle’s domain.