A slot is a dynamic placeholder that waits for content. A slot can be filled with content via a scenario (which specifies the content to feed the slot) or through a renderer (which specifies how to present that content to the viewer). A slot cannot contain more than one type of content.
The word slot comes from the idea that there is a narrow opening into which something else can be fitted: “a bolt, bar, or similar fastener, especially on a door or window”; “the job or position of chief copy editor” (from late Middle English slott, from Proto-Germanic *slutila- “bolt, lock, key”), and, also from 1888, the opening in a machine for a coin or other item to be inserted. The word is also used of the slot in an airplane’s wing that accommodates the landing gear.
In a modern electronic slot machine, it is the computer that decides whether or not you have won. It does this by reading a sequence of numbers from a massive spectrum, looking at how many matching symbols you have on a payline and comparing those to the number needed to trigger a bonus feature. It is impossible to determine the probability of a particular symbol appearing on a reel by watching it spin, but this is what makes slots so exciting and unpredictable.
A popular belief among slot players is that a machine which has gone a long time without paying out is due to hit soon. This is untrue, and the fact that a machine has not paid out for a while does not mean it is “due” to do so. This misconception is perhaps partly due to the placement of machines in casinos, with some being placed near the end of aisles, where it seems that they are more likely to be seen by other customers.