In the early days of computing, mainframes were huge computers that could fill an entire room or even a whole floor! As the size of computers has diminished while their power has increased, the term mainframe has fallen out of use in favor of enterprise server. You'll still hear the term mentioned, though, particularly in large companies to describe the huge machines processing millions of transactions every day, while simultaneously working to fulfill the needs of hundreds, if not thousands of individual users. Although mainframes traditionally meant a centralized computer linked to less powerful devices like workstations, this definition is blurring as smaller machines gain more power and mainframes get more flexible [source: IBM].
Mainframes first came to life in the post-World War II era, as the U.S. Department of Defense ramped up its energies to fight the Cold War. Even as servers become more numerous, mainframes are still used to crunch some of the biggest and most complex databases in the world. They help to secure countless sensitive transactions, from mobile payments to top-secret corporation information [source: Alba].
Indeed, IBM, one of the world's most enduring makers of mainframes for more than half a century, saw a spike in mainframe sales in 2018, for the first time in five years. That's in part because mainframes can pack so much calculating muscle into an area that's small than a rack of modern, high-speed servers [source: Hall].