The Billboard Hot 100

The Billboard Hot 100 is the music industry standard record chart in the United States for singles, published weekly by Billboard magazine. Chart rankings are based on sales (physical and digital), radio play and online streaming.

The weekly sales period was originally Monday to Sunday, when Nielsen started tracking sales in 1991, but was changed to Friday to Thursday in July 2015. Radio airplay, which, unlike sales figures and streaming data, is readily available on a real-time basis, and is tracked on a Monday to Sunday cycle (previously Wednesday to Tuesday). A new chart is compiled and officially released to the public by Billboard on Tuesdays. Example:

The first number one song of the Hot 100 was "Poor Little Fool" by Ricky Nelson, on August 4, 1958. As of the issue for the week ending on May 20, 2017, the Hot 100 has had 1,064 different number one hits. The current number one song is "I'm the One" by DJ Khaled featuring Justin Bieber, Quavo, Chance the Rapper and Lil Wayne.

Prior to 1955, Billboard did not have a unified, all-encompassing popularity chart, instead measuring songs by individual metrics. At the start of the rock era in 1955, three such charts existed:

Although officially all three charts had equal "weight" in terms of their importance, many chart historians refer to the Best Sellers in Stores chart when referencing a song's performance prior to the creation of the Hot 100; until the start of the rock era in 1955, radio was still in its Golden Age and centered around spoken-word programs instead of music radio, and physical record sales were still the dominant indicator of a recording's popularity. On the week ending November 12, 1955, Billboard published The Top 100 for the first time. The Top 100 combined all aspects of a single's performance (sales, airplay and jukebox activity), based on a point system that typically gave sales (purchases) more weight than radio airplay. The Best Sellers In Stores, Most Played by Jockeys and Most Played in Jukeboxes charts continued to be published concurrently with the new Top 100 chart.

...
Wikipedia