Looks like coolant temp sensor to me. GM part number 10096181 Delco #213-815. Wells part number SU-1302 from A-Zone.
It would help if you at least listed your vehicle in your sig.
A forum community dedicated to all General Motors makes and models owners and enthusiasts running the 3800 series engine. Come join the discussion about performance, modifications, classifieds, troubleshooting, builds, tunes, superchargers, maintenance, and more!