Defined in a wonderful article by Sherri Johns on Coffee Geek, she describes a Barista Jam as follows:

“So what is a Jam? The premise is modeled after a musician’s jam session. That informal event is usually a group of musicians who get together on a regular basis to play tunes... [but] Rather than musicians grouped together talking about riffs, scale changes, half notes and block cords, we have Baristi talking about shot times, tamping techniques, espresso blends and grinding burrs. Musical axes have been replaced with tampers, steam pitchers and beans. If this sounds like a science experiment to you, it is a little bit like one. Lab coats have given way to aprons. Loosely defined a jam is: anytime 2 or more Barista come together over an espresso machine to challenge themselves, or any time there is a gathering of several Baristi and machines all under one roof.”