What is this part?

This is the Fast Idle or Idle Air Control Valve. Depending on the setup it does act as the choke by sensing the coolant termperature. That is why it is part of the coolant circuit.
Some vechicels have the Fast Idle, (which is more like a choke) and the Idle Air Control, (which open the throttle more as load increases), as a unit or seperate parts. I cannot recall of hand on the Pathfinder. I think they are seperate. Make sure you get the right one because the Idle Air Control Valve is mcuh more expensive.
Purchasning a Haynes Manual or the online Mithcell1 manual offered through this sight will help identify the part.
Let me know if you have any other questions and I wilkl keep any eye on this post.

Ive inspected the part more closely, and looked at the Idle control valve online. They do not look to be the same, and I am not able to find a fast idle valve for this vehicle. The part is actually housed in the throttle body. Ive added a few pictures, one that shows the location in the throttle body and 1 or 2 in which I point the part out with a screwdriver. I appreciate your help entirely.

Fast Idle is probably the wrong name. That is kinda a Honda term. I looked around and could not find a picture of it, but it is the equivelent of a choke. I think it is called a Idle Control Solenoid, or Valve.
It kicks up the trhottle when the engine is cold.
Hope this helps.

I finally found an engine diagram and located the part. Its listed in the parts catalog as part # 16391-31U00. Its listed as ELEMENT KIT -THERMO. Its a temperature operated idle control. I guess it leaking is a pretty common thing with Nissan's that have high miles. Thanks for the help.

I am glad to hear you found the part and got the probleminformation you needed.
If you ever need anyting again, don't hesitate to come back.
Take care.

Dr, C