The history of radio has been of constant improvement in quality and in making it smaller and smaller. From big cumbersome radio sets the size of a desk, radio models have continuously been shrinking and shrinking to the point that some radio units are just added features of other devices, like clocks and wrist watches and cell phones. Since most radio stations are now also streaming their broadcasts on the internet, it is fair to say that radio has also invaded the world wide web.
Strictly speaking a portable radio is what we are familiar with. It’s what we grew up seeing. What the term means is that you can take your radio anywhere you want to. Except for stereo and discrete hi-fi equipment, most radios are portable.
Portable radios really started to boom or started to shrink with the invention of the transistor. Transistorized radios did away with vacuum tubes which needed more power to run. With smaller parts, and a smaller power requirement, portable radios shrunk further when integrated circuits came along.
Whichever form factor portable radios take, it sure is a good thing that you can take it with you wherever you go.