Interestingly, googling the term does not throw up results beyond the papers I originally found it in - related to dark matter detectors like XENON-100. I assume it's the live time of the experiment (which I'm also not sure of the definition for - the amount of time it was taking data, I think?), converted into days. Is that correct? If anyone could confirm this and give a complete definition, or point me in the direction of resources defining it, I'd be very grateful.
Here's where I originally came across the term: XENON100