Definition
Any other aircraft that a traffic avoidance system (such as TCAS or TAS) detects within its surveillance range and evaluates as a potential collision threat to the host aircraft.
Plain English
Another aircraft nearby that your traffic system has spotted and is watching to see if it might come too close.
Context Anchor
Seen in traffic avoidance system displays and alerts, where nearby aircraft are shown in relation to your aircraft.
Derivation
From Latin 'intrudere' (to thrust in). In everyday speech 'intruder' suggests someone unwelcome or hostile. Here it simply means an aircraft entering your system's monitored airspace -- no hostility implied.
Why Pilots Care
Intruders are the starting point for traffic alerts; the system evaluates each one to decide whether it needs a resolution advisory to prevent a collision.
Grounding Statement
Picture your aircraft at the center of the display; an intruder aircraft is another aircraft that appears close enough for the system to call your attention to it.
Intuition Check
Intruder does not mean enemy aircraft or someone breaking a rule. It simply means another aircraft the system is tracking because it could affect your flight path.
Example Sentence 1
The TCAS display showed an intruder aircraft 500 feet below and closing, prompting a traffic advisory.
Example Sentence 2
Even though it posed no risk, the system still marked the other plane as an intruder.