A slot (plural: slots) is a narrow opening in something, used for holding or receiving. The word slot is also used for a position or assignment in a series or cycle, as well as an opportunity or vacancy. Examples of this include a job, time slot on a TV program, or a place in a line. The word is derived from the Latin word for “slit” or “hole”. The American Heritage(R) Dictionary of the English Language, Fifth Edition
The first slot machines used revolving mechanical reels to display and determine results. Later, a computer system was added to make the odds calculations. These are now done by a random number generator, or RNG, that goes through millions or billions of numbers at all times. When a player hits the spin button, the machine stops the RNG at a certain point. This is why modern slot machines look like the old ones, with spinning reels and music, but in reality are purely gambling devices.