An automated security orb warning in-world. Displayed via the Linden Lab viewer.
A security orb is the generic name for an automated security-system used by many residents in Second Life to prevent trespassing on private property. Although security orbs vary in their specific feature-lists (according to its manufacturer), all such devices are designed to automatically eject avatars (usually after a warning period) who are not on the owner's white list.
Background
Typically, once set up by a resident on their land, a security orb will scan for potential intruders and if detected will issue the trespasser with a pop-up warning message before ejecting them from the land. Many orbs are set to provide the trespasser with a period of time to leave (usually 10-30 seconds), however, some have been known to immediately remove the intruder on detection. The range covered by an orb can vary depending on the whims of its owner; from a small parcel of land surrounding a single building, to an entire region - sometimes stretching hundreds of meters into the sky.
Orbs are generally considered a serious obstacle for Second Life pilots, as contact with one will potentially eject the aviator (and his/her passengers) from the land - and their aircraft. While orbs are commonplace on densely populated mainland continents such as Sansara and Heterocera Atoll, their presence is widely frowned upon in private estates popular with boating and aviation enthusiasts, with some estates such as Sailor's Cove regulating their usage by tenants. In addition, to reduce encounters with orbs, a variety of detector heads-up displays (HUDs) have been created by developers to warn travellers of nearby dangers.