The Top Forty or Top 40 is a music industry shorthand for the currently most-popular songs in a particular genre. When used without qualification, it typically refers to the best-selling or most frequently broadcast popular music songs of the previous week. Top 40 became the dominant radio format of the 1960s, 1970s, and 1980s.

The term "Top 40" for a radio format appeared in 1951.

The Top 40, or top forty, whether surveyed by a radio station or a publication, was a list of songs that shared only the common characteristic of being newly released. Its popularity coincided with the rapid changes in recording technology in the 1950s and 1960s. In 1954, the recording industry agreed upon a standard recording format for higher fidelity music, so any new record player could play any new record. Also in that year, new single records were released on 45 rpm records, and the Top 40 thereafter became a survey of the popularity of these records (and their airplay on the radio). Tape recording had become perfected, allowing artists more freedom as they composed songs, especially novelty songs.

A top (also called spinning top or spintop) is a toy that can be spun on an axis, balancing on a point. This motion is produced in the most simple forms of top by twirling the stem using the fingers. More sophisticated tops are spun by holding the axis firmly while pulling a string or twisting a stick or pushing an auger as shown in the pictures on this page. In the kinds with an auger, an internal weight rotates, producing an overall circular motion.

The top is one of the oldest recognizable toys found on archaeological sites. Spinning tops originated independently in cultures all over the world. Besides toys, tops have also historically been used for gambling and prophecy. Some role-playing gamers still use tops to augment dice in generating randomized results; it is in this case referred to as a spinner.^{[citation needed]} A thumbtack may also be made to spin on the same principles.

The action of a top relies on the gyroscopic effect for its operation. Typically the top will at first wobble until the shape of the tip and its interaction with the surface force it upright. After spinning upright for an extended period, the angular momentum, and therefore the gyroscopic effect will gradually lessen, leading to ever increasing precession, finally causing the top to topple in a frequently violent last thrash.