Expansion slot

In computing, the expansion card, expansion board, adapter card or accessory card, is a printed circuit board that can be inserted into an electrical connector, or expansion slot on a computer motherboard, backplane or riser card to add functionality to a computer system via the expansion bus.

An expansion bus is a computer bus which moves information between the internal hardware of a computer system (including the CPU and RAM) and peripheral devices. It is a collection of wires and that allows for the expansion of a computer.

Even vacuum-tube based computers had modular construction, but individual functions for peripheral devices filled a cabinet, not just a printed circuit board. Processor, memory and I/O cards became feasible with the development of integrated circuits. Expansion cards allowed a processor system to be adapted to the needs of the user, allowing variations in the type of devices connected, additions to memory, or optional features to the central processor (such as a floating point unit). Minicomputers, starting with the PDP-8, were made of multiple cards, all powered by and communicating through a passive backplane.

The first commercial microcomputer to feature expansion slots was the Micral N, in 1973. The first to establish a de facto standard was the Altair 8800, developed 1974-1975, which later became a multi-manufacturer standard, the S-100 bus.Many of these computers were also passive backplane designs, where all elements of the computer, (processor, memory, and I/O) plugged into a card carge which passively distributed signals and power between the cards.

...
Wikipedia