A slot is a container that can hold and manage dynamic items. A slot can either be passive and wait for content (as in a placeholder) or active, where it calls out for content from a repository. Slots work with scenarios and renderers to deliver the final content on a page.

From Wordnik:

A notch, groove, or opening in which something is situated, such as a keyway in a machine or the slit for a coin in a vending machine. Also figuratively: a position in a series or sequence, or the position of one element relative to another. [OED]
A slot in a casino game is a specific spot on a spinning reel that represents the opportunity to land a winning combination of symbols, which will trigger some bonus feature or payout. Slot machines have evolved greatly since their initial invention in the 19th century, incorporating many of the latest technological innovations into their mechanics and gameplay. These include touch-screen technology, which has revolutionized the way slots are played and incorporated into other casino games. Slots can be found in all shapes and sizes, with a range of themes, jackpot levels, and payout options. In addition, players can choose from a variety of different types of slots, including progressive ones that are linked to other machines and increase their jackpots with every spin. Some offer multiple winning lines and high payouts, while others feature Wilds that can unlock bonus features and other exciting elements. It is important to pick a machine that matches your play style, whether you prefer simple machines with a single payout line or flashy ones that have a wide array of bonus features.