The transliteration of the Cyrillic "ШKBAЛ", literally "squall" or "storm" in Russian, this word is the code name of Russia's 200-knot super-cavitating torpedo.
First, a little background on the theory of cavitation: when fluid travels across a surface at high enough speed, Bernoulli's Law tells us that its pressure is reduced at the contact point. If the pressure is low enough, gaseous elements dissolved in the fluid--like the dissolved oxygen in your aquarium--come out of solution and form cavities. Think of them as bubbles for now. For years, American fluid dynamicists have worked to eliminate cavitation, because propellers cause cavitation. The noise of these bubbles popping into existence and then collapsing again when they leave the flow is significant, and because they're less dense, they reduce the propeller's efficiency. If a non-cavitating propeller were developed, you can bet the US Navy would jump on it.
The Russian viewpoint of this problem is different: cavitation, to them, is a physical phenomenon that reduces any surface's drag with the surrounding fluid, and also happens to cause noise. Instead of trying to eliminate it, they have spent years trying to utilize it. Ever since the great Sergei Pavlovich Korolev's golden days, the Russians have been fascinated with the liquid-fuel rocket and more recently, its cousin the solid-fuel rocket.
At some point in the 60s, they cracked the code, and began testing (and some sources suggest deploying!) a torpedo whose specially shaped nose caused cavitation. The cavity builds up as the cavitation torpedo reaches sprint speed, and then small holes near the nose fire inert gases into the cavity, expanding it to the entire torpedo's length. A solid-fuel rocket motor fires, and some of the exhaust gases also channel into the cavity. Once the torpedo is in a pocket of air, four skids deploy around it, and it skips like a stone off the inside of a tunnel of water. It travels at an unprecedented 200 knots--essentially, an underwater missile. For comparison, America's Mk 48 torpedo is cited as approaching 40 knots at sprint speed. Its guidance probably leaves a lot to be desired, but given time and funding, it's entirely conceivable that these will replace or at least augment current stocks of "old fashioned" propeller torpedoes in navies rich enough to afford them.
It's going to change the way wars are fought at sea; it could be used to nuke an entire carrier group, or a port, if equipped with a physics package. It could be an engagement-breaker for the otherwise-inferior Russian subs. No American sub can outrun it, so once fired, the sub crew would have to evade, cutting the wires on any torpedo in the water. Since no engagements have been fought with the cavitation torpedo, no countermeasures exist. Theoretically speaking, some of the possible solutions include diving deep (to reduce the nozzle efficiency and possibly collapse the cavity) or surfacing (to cause the torpedo to breach and lose water contact on a control surface). Running away is almost guaranteed to fail.
Other interesting facts about the Shkval