Popular is a very broad style term generally encompassing any style not included in classical music, although some would also exclude folk, world, and ethnic music. Music that can be characterized another way such as jazz or country will usually be grouped under those names even though it will also be "popular" as opposed to classical. "Popular" is also the default name for non-classical music that does not fit well into any other style, with decided preference for sounds that sell well.
From the 1890s popularity was measured by sheet music sales. Record sales took over in the 1920s, as popularity was documented and influenced by US and UK radio programs as well as charts in magazines such as Billboard and Cashbox. As digital distribution replaced compact disc sales post-2000, measurement changed but sales success remained the first requirement for music to be considered popular.