Locations overview US definitions

Kind of US Bases with official definition

Main operating base (MOB)

Official definition: A permanently manned, well protected base, used to support permanently deployed forces, and with robust sea and/or air access.
Wikipedia EN: https://en.wikipedia.org/wiki/Main_operating_base
DE: https://de.wikipedia.org/wiki/Main_Operating_Base

Forward operating base (FOB)

Official definition: An airfield used to support tactical operations without establishing full support facilities. The base may be used for an extended time period. Support by a main operating base will be required to provide backup support for a forward operating base. Actually
Wikipedia EN: https://en.wikipedia.org/wiki/Forward_operating_base

Forward operating site (FOS)

Official definition: A scalable, ‚warm‘ facility that can support sustained operations, but with only a small permanent presence of support or contractor personnel as opposed to a FOB or MOB. A FOS will host occasional rotational forces and many contain pre-positioned equipment.
Note: Synonym defined as Forward operating location (FOL)
Wikipedia EN: https://en.wikipedia.org/wiki/Forward_operating_site
DE: https://de.wikipedia.org/wiki/Forward_Operating_Site

Cooperative security location (CSL)

Official definition: A CSL is a host-nation facility with little or no permanent U.S. personnel presence, which may contain pre-positioned equipment and/or logistical arrangements and serve both for security cooperation activities and contingency access.
Wikipedia EN: https://en.wikipedia.org/wiki/Cooperative_security_location
DE: https://de.wikipedia.org/wiki/Cooperative_Security_Location

Geographically Separate Unit (GSU)

Official definition: A GSU is a base that is physically separate from, yet not autonomous of its „parent“ base. GSU is corresponding with Further locations or listed among BSR entries in VisualBases
Wikipedia EN: https://en.wikipedia.org/wiki/Geographically_Separate_Unit
Note: This is no clear definition concerning the DoD handling within the Base Structure Report.

The US Base Structure Report

VisualBases is not a tool to deliver a total figure of all foreign and overseas military bases. But it is necessary to compare the figures which appear specific to a country with the official data. Because of the dominance of US military bases the annual US report „Base Structure Report – BSR“ is of special interest. This reports presents total figures of „Installations“, which are not matching the amount of single location. The official definition is:
„Installation: A military base, camp, post, station, yard, center, homeport facility for any ship, or other activity under the jurisdiction of the Department of Defense, including leased space, that is controlled by, or primarily supports DoD’s activities. An install may consist of one or more sites.“

Extracts from a BSR you can see here: http://files.visualbases.org/pdf/BSR_Extracts_FY2018.pdf

Definitions used by David Vine

The following definitions are used in the spreadsheet list of David Vine.

Base

⇒ generally, means any „base site“ listed by name in the Pentagon’s annual Base Structure Report (to be listed by name it must have more than 10 acres or a value of more than $10 million) and any other significant military installation outside the 50 states and DC with US military facilities and more than around 200 US military personnel (military, civilian, or contractors), as reported by at least one or more high quality government, news, or academic source;

Lily Pad/Small Base

⇒ a small military installation, including a base site counted in the Pentagon’s annual Base Structure Report as an unnamed „other site,“ which by the Pentagon’s definition are sites consisting of less than 10 acres or a value of less than $10 million. I also include any other cooperative security location, forward operating location/site, or other location outside the 50 states and DC with US military facilities, as reported by at least one or more high quality government, news, or academic source; and any installation that falls under the „base“ definition but where it would feel intellectually dishonest or misleading to present it as such. N.B. I have thus excluded places where a cooperative security location agreement has been reported (e.g., by CRS 2011) but I have no evidence there is any US military infrastructure or personnel on the ground. A slightly more expansive definition of a lily pad would include these countries (Botswana, Namibia, Sierra Leone, Tunisia, Zambia).

Unconfirmed Base

⇒ where US officials have investigated or negotiated the creation of a lily pad or other base as reported by at least one source and/or a location where a lily pad has been reported by at least one source that is not of a quality that gives me confidence to list it.

US-funded Host Nation Base

⇒ no known facilities designed explicitly for US use, although US personnel may have access to and/or use facilities paid for in whole or in part with US taxpayer funds. Those listed here are far from exhaustive; most here are in Latin America. Afghanistan alone would have many, many such bases.

Closed

⇒ base closed and to which the United States may retain access in some cases.

More info: Definitions by David Vine, i.e.: CSL / „Lily Pads“) – U.S. naming conventions in WIkipedia