Pop is short for popular, and it's remained the defining term for the ever-changing music favoured by the public. Although not specifically applied until the middle of the 20th century, pop music as such can be traced by a few decades before that.
The Early Days
You could say that the songs of music hall were the first real pop songs, written by professionals and widely performed for audiences. That dates back to the Victorian era, when a performer needed a catchy, identifiable song.
Things changed with the advent of recording, early in the 20th century. With that, music had the chance to be much more widely disseminated. Records, played at 78 rpm on wind-up gramophones, were relatively cheap.
In America, that led to a breed of professional songwriters in New York who wrote pieces intended to be recorded and sell well - Tin Pan Alley. They were largely hacks, but did produce some beautiful material. London had its own equivalent as the music business became centred on Denmark Street in the West End.
The Crooners
The first major pop stars as such were the crooners of the 1930s and '40s. Bing Crosby sold millions of records, as did Frank Sinatra (arguably the first modern pop star, with screaming teenage female fans - the bobbysoxers), and in Britain, Al Bowly.
They recorded and performed with full orchestras in the main style of the day. But there were other vocals groups, such as the Mills Brothers and the Inkspots, whose harmonies set the standards for those aspiring to fame.
With the style known as swing, big bands also came into their own, with tunes like Glen Miller's "In The Mood" becoming standards.