When one thinks about a portrait lens you tend to think of a fast sharp 85mm f/1.4. Its certainly one focal length that many manufactures have a good version off.
Its a focal length that I have been considering purchasing for sometime but as I own a 70-200mm f/2.8 zoom and a 105mm I have been managing quite happily.
For fun and personal work I often shoot portraits with my little Leica M. I find the Leica fun to use and is useful when working with less professional models as the camera is less intimidating.
For full length portraits a 50mm works well, but going wider you run a risk of distorting the figure and care is needed.
In my work when the working conditions are tight I have gone as wide as 24mm. With wider lens you can use to with care to make the legs look longer, shooting from a low angle.
If you have a search on the web for well know photographers you can see some portrait photographers have produced some amazing images with lens as wide as 21mm.
So is any lens a portrait lens?
In the past many portrait lens were designed for low contrast and certainly less sharp then modern lens, a more forgiving lens for the human face.
Its posible now to buy lens that wide open are soft and dreaming but then stopped down a couple of stops are as sharp as any top lens.
Now the look of a lens is often forgotten as Photoshop is used to give the effect one wants, but its nice to get as much done in camera and just leave some basic editing for post production.
But in answer to the question, yes any lens is a portrait lens, generally 55mm to 135mm traditionally but I have seen great portraits from 21mm lens to 300mm and even the 500mm mirror lens.