Transmission Control Indicator Lamp (TCIL)
WARNING: This page does not describe the selected car, but rather 32 other vehicles, including the 2010 Mercury Milan, 2010 Mercury Mariner, 2010 Mercury Grand Marquis, 2010 Lincoln Town Car, and 2010 Lincoln Navigator L. However, it is still accessible from the selected car via links, so may be relevant.
The TCIL is an output signal from the PCM that controls the lamp on/off function depending on the engagement or disengagement of overdrive.