Pure - being no chemicals...like chlorine, fluoride, aluminum residues (used to make water clear) ...such as found in municipally-treated water supply.
Some years ago, we had a running discussion on 'good' water, bottled water, designer bottled water that contained too much calcium and found to be a promoter of AF, and then the home filter systems which aren't typically helpful compared with the more expensive filtering systems including Reverse Osmosis water...and then the need to add back in essential minerals.
If I can find that link or something in my files, I'll repost it here.
Jackie