||It has been suggested that this article be merged with hit parade. (Discuss) Proposed since September 2012.|
Many different criteria are used in different charts, including sales of records, cassettes and compact discs, the amount of radio airplay, and (since the introduction of the technology) the number of downloads.
Some charts are specific to a particular musical genre and most to a particular geographical location (although download charts are not easily pinned down in this way). The most common period of time covered by a chart is one week with the chart being printed or broadcast at the end of this time. Summary charts for years and decades are then calculated from their component weekly charts. Component charts have become an increasingly important way to measure the commercial success of individual songs.
Chart hit 
A chart hit is an extremely popular recording, identified by its inclusion in a chart that uses sales or other criteria to rank popular releases. Chart-topper and related terms (like No. 1 hit, top of the charts, chart hit, and so forth) are widely used in common conversation and in marketing, and are loosely defined. In North America, the weekly charts from Billboard magazine are most often referenced (quite often internationally, as well), particularly the Billboard Hot 100 singles chart and Billboard 200 album chart, although there are many other charts and sources. Because of its value in promoting recording artists and releases, both directly to the consumer, and by encouraging exposure on radio, TV other media, chart positioning has long been a subject of scrutiny and controversy. Chart compilation methodology and data sources vary, ranging from buzz charts based on opinions of various experts and tastemakers, to charts that reflect empirical data such as retail sales. Therefore, a chart-topper may be anything from an "insiders' pick" to a runaway seller.
In 1988 a California radio engineer (B Golden) examined a database of CHR (Contemporary Hit Radio, i.e., Top 40) charts from Radio & Records, then a popular trade publication, and observed that over a 4-year sample period from 30 Sep 1983 to 25 Sep 1987 the most frequent peak position for songs that charted at all was #1, followed by a noisy descent zigzagging all the way down to #40, the most uncommon. Over 12% of songs, nearly 1 in 8, made it to #1 and nearly half the charted songs peaked in the top 10. It was suggested that this is a common feature of all such charts, and emerges from the fact that at each week’s shuffling, the closer a slot is to the top the smaller the number of songs descending from above available to compete for it.
Other terminology 
There are several other commonly-used terms when referring to a music/entertainment chart or the performance of a release thereon.
The term new entry is commonly used to denote a title which is making its début appearance in that chart. This is applied to all charts, for instance a track which is outside the Top 40 but which later climbs into that level of the chart is considered to be a 'new entry' to the Top 40 that week. In most official charts, tracks have to have been on sale for a period of time in order to enter the charts; however, in some retailers' charts, new releases are included in charts as 'new entries' without a sales history in order to make them more visible to purchasers. A real new entry is a title that makes its first début on the chart no matter how many positions officially the chart actually is. In the UK the official published chart is a Top 100 although a new entry can take place between positions 101-200. The Top 40 is only used for radio to shorten the play-lists.
The term re-entry is used if a track which has previously entered a chart falls out of that chart and then later re-appears in it. This may come about if a release is reissued or if there is a surge of interest in the track. Generally any repeat entry of a track into a chart is considered a re-entry, unless the later version of the track is a materially different recording or significantly repackaged (such as Michael Jackson's "Thriller 25"), where the release would normally be considered separate and thus a "new" entry.
The term climber is used to refer to a release which is going higher in the chart week-on-week. Because chart positions are generally relative to each other on a week-to-week basis, a release does not necessarily have to increase sales week-to-week to be a climber, as if releases ahead of it decline in sales sufficiently they may slip below it. By the same metric, not all week-to-week sales increases result in a climber, if other releases improve by a sufficient amount to keep it from climbing. The term highest climber is used to denote the release making the biggest leap upwards in the chart that week. There is generally not an equivalent phrase for tracks going down the chart; the term "faller" is occasionally used, but not as widely as 'climber'.
The terms top 10, top 20 and so forth are used to determine the relative success of a release. For instance, a track may be referred to as a 'top 10 hit' if it reaches a position between 1 and 10 on the singles chart, as a 'top 20 hit' if it reaches between positions 1 and 20, and so on. The most commonly known chart is the 'top 40' widely used by the media in various territories, though it is common for longer lists to be produced for or by the music industry. For example, in the UK, the Official Charts Company produces a top 200, although various media only publish shorter lists.
The term flop is a title that never reaches an official chart position. A relative flop is a term given to a title by an act that usually has big hits then only a minor hit in between.
The term one-hit wonder is for an act that appears on the chart just once, although the term true one-hit wonder was the term given by Guinness Book of British Hit Singles & Albums (and also the Billboard book Top Pop Singles) is an act that has one number one hit and nothing else on the chart ever. If an act appears in some other form, (for example) a solo act that appears with a band or with other acts then these are taken separately.
See also