I asked my friend about a band that I'd missed earlier that evening - was it jazz? "What is jazz?" she said, with a shrug. Well, you hope you know it when you hear it. Festivals like this one move easily from classical fusion to world music and DJs, but it's jazz that gives it its soul.

The question hovers throughout the festival. Principle 3, five percussionists and a bassist from Gateshead, could keep the crowds happy at a classical or world music bash, but they kicked off their set with Chick Corea's Latin Jazz classic La Fiesta. Jumping around their massive percussion collection on the Freestage, Principle 3 kept the energy levels high with a nicely asymmetric selection of originals, making good use of tuned instruments such as marimba, vibraphone and steel pans.

There's no doubt about Eivind Aarset's jazz credentials; the guitarist is following in the nu-jazz footsteps of his old boss Nils Petter Molvaer, with a confident, inventive integration of digital squiggles and loops into masterful improvisation. However, his band also have a touch of the power trio, threatening to burst into rockist cliches on the more melodic, anthemic numbers. Gary Moore should watch out.

Hiring the Vienna Art Orchestra was a stroke of genius. At the Purcell Room, the big band overcame the acoustics of that difficult venue to play a scorching set, with brilliant Mathias Rüegg originals such as the madly rapid Off Beat Berlin on the Beat and Tango from Obango (featuring some vaudeville business from the lead trumpeter and bassist). Rüegg also gave us a dense, satisfying arrangement of Mongo Santamaria's famous Afro Blue. The VAO redefine the concept of a "well-drilled" band - they know these clever arrangements so well that they can reach out to new levels of intensity and spirit. And that's jazz.

· The London jazz festival ends tomorrow. Details: serious.org.uk or bbc.co.uk/radio3/