A planetary system (or star system) is a set of gravitationally bound non-stellar objects in or out of orbit around a star or star system. Within a system, there is often a habitable zone. Within this zone, one or more planets may lie which can support life. However, life, particularly non-humanoid life, can flourish worlds far outside this zone.
In Star Trek Online, systems can refer to any entry point in sector space, such as a space station or an anomaly, independently from its nearby star.