Wild yeast; it's a term used for levain, sourdough in general. But I just started using a different ferment; wild yeast or jefe vorteig (German for pre-ferment dough). I found this on Bernd's Bakery, a most interesting Swiss bakers site.
I have used it a few times already. It gives a good oven spring to bread when your levain is on the-down- low. Bernd suggest's rye, but I had none around so I used barley, worked only after four day's even in the winter coolness of my kitchen. Bernd's recipes often uses them in conjuction or alone to make different breads. I've fooled about with it, myself, so far with wheat, next maybe rye or buckwheat.