A hit single is a recorded song or instrumental released as a single that has become very popular. Though it sometimes means any widely played or big-selling song, the term "hit" usually refers to a single that has appeared in an official music chart through repeated radio airplay or significant commercial sales.
Historically, before the dominance of recorded music, commercial sheet music sales of individual songs were similarly promoted and tracked as singles and albums are now. For example, in 1894, Edward B. Marks and Joe Stern released The Little Lost Child, which sold more than a million copies nationwide, based mainly on its success as an illustrated song, analogous to today's music videos.
In the United States and the United Kingdom, a single is usually considered a hit when it reaches the official Billboard Magazine's Hot 100 or the Top 75 of the 'UK Singles Chart' and stays there for at least one week. The Guinness Book of British Hit Singles has used this definition since the 1970s. Some argue that reaching at least the top 100--since the Official Charts Company increased the chart size on their website on 23 June 2007--lets a single be considered a hit due to the increasing singles market after chart rules included download singles.
A hit single is variously called a number one hit, a top 10 hit, a top 20 hit or a top 40 hit, depending on its peak position. In the UK (where radio play is not included in the official charts), this doesn't completely reflect the song's popularity--as the weekly chart position is based solely on direct comparison with concurrent sales of other singles. It is, therefore, not uncommon that a single fails to chart, but has actually sold more copies than other singles regarded as hits based on their higher chart placement in a period of low sales. (This is also possible in the United States, or anywhere, with slow but steady sellers; a number of minor hits, especially those that are popular in specific genre, have earned gold certifications despite relatively poor pop chart performances.)
In the UK, the number of sales required to achieve a hit single steadily declined in line with a general decline in single sales until the early 2000s. The number has, however, recovered strongly with growth in official digital downloads. 2011 was a record year for UK singles sales. Actual figures vary considerably depending on the time of year. In 2010 a number one single usually sold around 100,000 copies per week; sales of around 30,000 were often sufficient to reach the top ten, and a single selling over 6,000 copies could make the top forty.
Whether or not a single becomes a hit depends on geographical location. Musical taste varies considerably, and singles that are not hits in their country of origin sometimes become hits elsewhere. For example, Blind To The Groove, released in 1998 by UK band Ultra, did not chart in the UK but became a top ten hit in Spain. Nicole Scherzinger is far more popular in the UK, than her native US, scoring a number one single in 2011 (Don't Hold Your Breath) in the UK, as well as other hits there, but with little success in the US as a solo artist.
Record companies often release collections of hit singles by various artists as compilation albums, such as the Now That's What I Call Music! series. Well-known bands and artists also frequently release collections of their most popular singles as Greatest hits albums.