A:Does he mean bivy bag. Because that's usually where the word bivy comes in. A bivy (or bivouac) bag is simply a water-resistant (often made from Gore-Tex) tub that you slide into-dressed, or in a sleeping bag.
I'm not wild about bivy bags, except as a superlight shelter of last resort. For almost all camping I prefer a tent. And besides, tents are so light these days that bivy bags really don't offer much advantage in the weight-saving department.
A really good solo shelter would be Sierra Designs' Light Year 1 ($170). It's a tube-style tent, meaning you stay out the nose and tail with rope, and two poles give the tent some shape. That saves you a pole over free-standing tents, and is a very sturdy and weatherproof design. Weighs only three pounds. It's not a big tent, but it's tall enough to sit up in, read a book, change a shirt, and so on.
REI has come out with some great tents in recent years, such as their really nice solo-shelter Quarter Dome T1 ($199). It's a touch heavier than the Light Year (3 pounds 6 ounces). But it's free-standing-which has its conveniences-and has much more interior volume. So it's a more comfortable tent to "live" in. Has a generous vestibule, too, for storing gear.
Marmot's EOS 1P ($225) manages to be both freestanding AND as light as the Light Year (3 pounds, depending on how many stakes and other accessories you pack). Like the REI tent, it has an arched design that is pretty roomy for a solo tent, with a good-sized vestibule on the door side. Marmot puts a lot of thought into their tents and this one shows it, with lots of nice little touches like jingle-resistant zipper pulls.
Finally for the ne plus ultra in light weight, there is always the Outdoor Research Aurora Bivy ($199). It's just a fancy Gore-Tex bag, but perfectly weatherproof and at 1 pound 8 ounces, as light as it gets.