(1) Place of slow water.
Since this is usually well below ground level, and very, very quiet, you will never see or hear it, but it's still important, not so much in relation to hiking but for drinking.
An aquitard is something that retards or slows the flow of groundwater, like a layer of clay, or like a layer of rock that is permeable but not by much, or densely packed fine sand.
An aquitard forms a barrier through which water can leak though not well, but through which it can leak, slowly.
Underground water will flow along an aquitard as well as percolate through it, or can be stored within one.
Two aquitards with a more permeable layer between them (plus a slope) make for an artesian system.
(2) Jules Leotard was a French trapeze artist who wore a distinctive costume that came to be named after him.
Less well known was his younger brother Aquil, the swimmer, who invented the swimming suit, possibly because he had a small peepee and wanted to hide it, or maybe not.
Maybe it was because he was a natural showman and needed a distinctive brand, which nudity was not.
At any rate he was famous and dashing, and splashing, as befits a champion aquanaut in the days before game shows and reality television, when entertainment was anything you could do that wasn't ordinary the way that growing potatoes was ordinary, or the way that chewing was ordinary, or the way that being overwhelmed by a tide of diseases and parasites, succumbing, and being dumped into a hole in the dirt and forgotten was ordinary.
On the contrary, wearing a powder blue, skin-hugging suit and diving from a dangerously high platform was not ordinary, and Aquil became famous and was envied by many, even some with much larger peepees, because of his odd yet compellingly manly flamboyance and his ability to sail through the air, powered only by gravity, controlled only by his will, protected only by his paper-thin aquitard (as it came to be known).
But after a while this all got old and he was forgotten anyway, and now if you want an aquitard you have to make your own, but people will only laugh at you and make rude gestures, though it can be fun to hike in one (camo version) if you are way, way out there and very much alone, which few of us can manage with aplomb.
(3) Someone who carries and uses a one pound, $150 water filter, stopping at every opportunity to spend a half hour or so pumping away furiously, filling bottle after bottle (at two pounds a quart -- even more per liter) in order to carry along lots of sparkly safe water, only to stuff the filter away inside a pack pocket where the clean end of the hose immediately comes into contact with the dirty end.
An aquitard frequently dumps excess water only to filter more of it to have the freshest available.
An aquitard is afraid to carry and use a one or two ounce chemical treatment because the big heavy expensive time wasting choice is always better.
(4) A layer of that dark mysterious inscrutable excessively hard underground rock stuff that is permeable, but not very much.
It forms a barrier that inhibits water's free expression of liberty and joy, and may be a component in the whole artesian conspiracy.
It has very little to do with backpacking other than that it helps to keep the landscape from sloshing around and leaking like an abused water bed as you walk over it.
Which could be disconcerting if it ever did happen.
But on the other hand this could be fun, possibly, if you were with the right group.
Too bad you never get invited to go with the fun kids though.
It really is.