The Reaper Program.

The Reaper program is a program that runs on all servers and workstations that will terminate login sessions that appear to be idle or hung. To determine this, the program uses a number of system measurements (NOT including actual cpu time used) to determine how long a connection has been idle. Connections that are active for more than 5 hours are considered to be hung, regardless of their current idle status

When a connection is determined to be idle, the reaper program kills all associated programs with that processes. This usually causes the user to be logged off.