We first heard the term "host" being used by fast teams involved in our best practice research project.
Host, as they defined it, is everything in an organization that is outside and around a team. The host can provision a team or it can interrupt it. Fast organizations design the host to "provide for" when resources, information, or "things" are needed by the team to be effective. For example, HP has always been good at this.
Slow organizations have interrupting hosts that place controls on teams, limit budgets, resources, information, and "things" -- efficiency is consumed by a team "working around" the system (or host interrupts). These interrupts are systemic and situational to projects. For example, the high walls of the marketing and engineering silos tend to be a systemic problem in most companies. It can also be situational and affect just a single project, these are easier to fix.
We use Force Field Analysis to identify the restraining forces and remove them to improve efficiency. Most people focus on the wrong stuff to improve efficiency, i.e. those "driving forces" versus the restraining forces. This concept is a foundation practice.
The HOST can also be the management hierarchy. They are the first line provisioners to the team and can be the first line unknowing "interrupters." The host can also be stakeholders (internal customers) and other dependent functions like product lines whose hardware on which your software runs (for example). Host can also be all of the support organizations that support the development effort; legal, contracts, purchasing, HR, other design groups, and so on.
The team is clearly the people that go to work every day and work on "project" things. The most effective development teams are dedicated (i.e. 100% allocated to a single project for an extended period of time). As resources get multiplexed beyond 2.5 their effectiveness drops off dramatically. Most teams don't have the luxury anymore to have this level of dedication and focus. The more you multiplex the less you get!