An extension housing seal is an oil seal located at the back end of the transmission on a rear-wheel or four-wheel-drive vehicle.
The rearmost part of a transmission, the place where the output shaft (and often the vehicle speed sensor) is located, is called the extension housing. The driveshaft connects to this part of the transmission by way of the slip yoke.
The extension housing seal, sometimes called a transmission output shaft seal, is pressed into the housing, and the driveshaft slip yoke is inserted through the seal. The seal stops transmission fluid from leaking out and prevents dust or moisture from getting inside.