Depending on context, the 170 foot-high cascade of the Niagara River between Lake Erie and Lake Ontario, or the eponymous American and Canadian cities that bracket it.
Niagara Falls, the geological formation, is believed to be about 12,000 years old, dating back to the most recent Ice Age. At the time, water flowing out of Lake Erie dropped about 35 feet (11 m) as it entered Lake Ontario. The force of the falling water-- about 35,000 cubic feet per second-- began to erode the underlying glacially deposited rock, and undercut the base of the falls, slowly moving it upstream (south) and raising the elevation. Naturally, as the falls got higher, the erosive effect of the water increased, hastening its retreat upstream. Today, the falls are located about 7 miles upstream from where they began, and have risen to a height of about 170 feet. In their wake, they left the Niagara Escarpment, a deep gorge (including rapids and a whirlpool) through which the river eventually makes its way to Lake Ontario.
Presently, the falls are comprised of two separate cascades, the 1,100 foot-long "American Falls," located along the eastern shore of the Niagara River, and the 2,200-foot "Horseshoe Falls" located along the western shore, completely within the Canadian border. Goat Island (connected to the American side by a small bridge) separates the two falls. Only about 10% of the total volume of the river-- 37 million gallons a minute-- flows over the American Falls, with the remainder flowing over the 180-degree arc of the Horseshoe Falls. Geologists estimate that the falls split only about 400 years ago.
Father Louis Hennepin, a French priest and missionary, is considered to be the first European to view Niagara Falls in 1678, although the Seneca indians had been living in the region for about 300 years by that time. By the mid-1700s, the French had established several settlements and forts along the Niagara River, and over the next 150 years, Niagara Falls grew into an active manufacturing center, with many industries tapping the river as a source of power.
At the same time, the falls also began to attract tourists. By the early 1900s, over a million visitors a year came to view the falls. It also began to attract daredevils. The falls' first, and still most famous, daredevil was a French acrobat named "Blondin," who crossed over the falls on a tightrope in 1859. Over the next few years, Blondin made dozens more crossings, each accompanied by more and more outrageous stunts, such as carrying his manager on his back, pushing a wheelbarrow across the tightrope, and stopping midway to fry an omlette on a small camp stove. Other dareveils followed in Blondin's footsteps (so to speak), crossing tightropes over the falls blindfolded, and, in the case of one female daredevil, wearing peach baskets on her feet.
The first person to perform the now-cliched stunt of going over the falls in a barrel was a schoolteacher named Annie Taylor in 1901. After surviving the tumble over the Horeshoe Falls and washing ashore at the base, Taylor's first words to her rescuers were "nobody ought ever do that again." But they did-- several dozen daredevils sought to replicate Taylor's stunt in various manners, but only about half of them managed to survive. All of their achievements, however, are overshadowed by the accidental accomplishment of 7-year old Roger Woodward. Woodward, his sister, and a family friend were fishing in the upper Niagara River, about a mile from the falls, when their boat capsized, and all three were swept downstream. Woodward's sister was pulled ashore by some bystanders, but Woodward and the family friend were swept over the Horseshoe Falls. Woodward survived and was spotted by the Maid of the Mist sightseeing boat and rescued, thus becoming the first-- and only-- person to go over the falls without protection and survive. (The family friend was killed.)
During the mid-1900s, Niagara Falls became one of the most popular honeymoon destinations in the world, its name becoming almost synonymous with romance. Hotels, giant observation towers, and other tourist attractions sprang up on both sides of the falls, although the Canadian side, by virtue of its superior vantage point, which allows tourists to take in the entire vista of the falls at once, always was and continues to be the more popular destination.
Although the power of the Niagara River has been tapped by industry for years, in the 1950s, authorities on both sides of the river began ambitious projects to harness the river's flow to generate hydroelectricity. Over the course of a decade, huge tunnels were dug from intake vents about 2 miles north of the falls, passing under the cities of Niagara Falls, New York and Ontario, and into large reservoirs downstream of the falls. Each reservoir, in turn, feeds huge hydroelectric dams that, combined, generate more than 4.2 million kilowatts of electricity, and supply power to much of the Northeastern U.S. and Canada.
Although unknown by most visitors to Niagara Falls, the diversion of water for electrical generating robs the falls of nearly a third of their water volume at all times, and by nearly 75% overnight while the reservoirs are refilled. (The falls are illuminated by colored lights until midnight during the summer, and the most severe diversion begins shortly after the lights are turned off. Walking by the falls after midnight is a remarkable experience, as the thunder of the falls is greatly muted.) As a result of this diversion, erosion of the falls, which had histoically averaged close to 3 feet upstream per year, has all but stopped, and the falls today move south at less than 3 inches every decade. Some citizens of the region advocate for a short period each year where the water diversion is stopped and the falls are allowed to return temporarily to their natural volume. Of course, such an act would result in a dramatic rise in water levels downstream, washing away docks and flooding lower lying areas, so naturally, it is unlikely to ever happen.
The falls have stopped flowing on two occasions. In March of 1848, a buildup of ice at the mouth of Lake Erie effectively halted the flow of water into the Niagara River. For two days, the falls were all but dry, allowing geologists and historians an unprecedented opportunity to explore the riverbed. The Army Corps of Engineers also diverted away all of the water flowing to the American side for several months in 1969, in order to study the effects of erosion on that side and to consider removing the huge boulders or "talus" that had accumulated at the bottom of the American Falls from rockslides over the years.
Niagara Falls, New York, on the other hand, is practically a ghost town, whose major attraction is Love Canal, a former residential neighborhood polluted with dioxin by Hooker Chemical Co. (now Occidental Chemical). Because it lacks the vistas of its sister city in Canada, and because of horrendous political corruption and economic incompetency, Niagara Falls, N.Y. has failed to fully capitalize on its assets and remains one of the most economically depressed cities in the Rust Belt. That being said, it actually has some beatiful parks and scenery, and is a nice, inexpensive place to live.
Niagara Falls, Ontario is Las Vegas writ small. Chockablock with kitschy tourist traps, a robust casino, dozens of hotels, and various other attractions, it has been able to fly where its neighbor across the border has faltered. It's an active and busy place, day and night, year 'round, and can't understand why the Americans can't get their act together.
(Many of the figures in this writeup were found at "Thunder Alley," www.iaw.com/~falls, an incredibly complete and comprehensive storehouse of information about the Niagara region.)