A photograph that one has taken of oneself, typically one taken with a smartphone or webcam and shared via social media.
Selfies have become a kind of pop-culture joke for many; Millenials taking and posting endless photos of themselves doing anything (or nothing). While it can smack of unthinking narcissism (if only because of the sheer volume of the output), it's not really that different from what Uncle Bob used to do with his Kodak Brownie camera every Thanksgiving and holiday; he tried to preserve a memory, freeze-frame a moment in time for himself or others to enjoy later.
Selfies appear more ego-centric, only because the taker of the photo also appears in it - but a lot of "selfies" feature groups of friends. Isn't that better than Uncle Bob only seldom appearing in family photos himself, because he was invariably the photographer?
The main difference is that via the smartphone many now have a camera available almost all the time, and the ability to make the images available publicly. Rather than poring over a dusty photo album years later to view the fading memories, they are immediately put out there (and perhaps just as soon forgotten).
It's not really a new phenomenon, we are just doing it a bit differently.