A picture taken of yourself, preferably with a recognisable backdrop and timestamp to prove that you are actually where you claim to be.
My girlfriend doesn't trust me and didn't believe that I was at my brother's house this morning... I had to send her an Alibi Selfie.
Coworker: Hey, the boss thinks that you lied about taking off early to get new tires for your car. You might be in trouble!
Me: Don't worry, all is fine.
Coworker: Dude, I'm worried.
Me: I've taken selfies of myself at various places, including a tire store. I sent her an alibi selfie in a tire store waiting room.
Coworker: Damn you're good...