The one I most like, and the one I'll be adopting for now, was this one - articulated by Todd Zaki Warfel:
"Information architecture organizes stuff to make it useful, findable, and actionable.
- For ecommerce sites, we organize products and descriptions to make the shopping experience better
- For content and news sites, we make finding the stories you're interested in easier
- For information systems (e.g. aviation systems) we organize gauges and displays to help reduce error"
I like the way this is constructed with the DIY examples at the end to help contextualise the definition for whatever audience, project, or discussion you may be having at the time. I know there are some holes in this - it doesn't address the fact that 'information architecture' is also a discipline or emerging community of practice. But, to be perfectly honest, I don't care. In 99% of cases, no-one I discuss IA with will care either. So I'll stick with this one for now.