Autonomous

Autonomous means existing and continuing as normal without assistance from others (primarily your empire/allies in dsotg). It can be used differently in two contexts:

Referring to worlds...
When used in reference to a world or worlds, it means to say that the world in question is capable of indefinite continuation of a role (usually its current role), without requiring any form of importing. Sometimes storage worlds are referred to as this, as are defensive installments with no imports. Worlds will low imports (light amounts of fuel ) are often referred to as lightly-dependent or near-autonomous. An example would be a world that can last 180 days with its current stock.

Referring to ships...
When autonomous is used in reference to ships (or a fleet /group of ships), it means that this ship, fleet or group has the capabilities can sustain itself for a very long period of time. Though this definition varies from person to person, this generally means a ship has mining capabilities to gain more or equal fuel in general flight then it uses whilst still flying adequately (1/2 cruise speed or so). This definition therefore sees usage in describing ships able-bodied for long range flights and attacks into enemy territory, though rarely do combat ships actually have the level of autonomousy expected in general. Ships which do not gain more fuel but instead are very cheap to run and have high currencies and investements from the empire or onboard can be described as autonomous, though in the strictest of senses this wouldn't really be true, and so sometimes the term near-autonomous or lightly dependent is used in reference.