Folllowing the latest blog post which looks at how academic careers can involve multiple activities / roles, I decided to find an actual definition of a portfolio career.

What I found, within 10 seconds, was an entire website dedicated to the concept! http://portfoliocareers.net/

It turns out that this website actually is linked to a book but on looking through their blog I did find this excerpt which I found very useful. http://portfoliocareers.net/tools/