What is the name of the long cable/plug attachment on the MacBook MagSafe charger?
The cable in question (pun intended) is called Power Adapter Extension Cable.
Overview
The 1.8-meter Power Adapter Extension Cable is an AC extension cord that provides extra length for your Apple power adapter. Use it with MagSafe and MagSafe 2 power adapters, 10W and 12W USB-A power adapters, and 29W, 30W, 61W, and 96W USB-C power adapters.
P.S.: The attached image is for illustration purpose and is obtained from the aforementioned hyperlink from the Apple Online Store.