A portable transformer plug is a device that allows you to use an electrical device designed for one type of electrical outlet in another type of outlet.

It typically consists of a plug on one end that can be inserted into an electrical outlet, and a number of sockets on the other end that can be used to plug in devices designed for a different type of outlet. 

Portable transformer plugs are often used for travel or when working in areas where different types of electrical outlets are used. They can be used to convert a standard North American plug to a European or Asian plug, or to convert a European plug to a North American plug. These are also called as travel adapters, plug adapters, or outlet adapters.