In-system programming (ISP)
Writing new firmware to a chip while it stays soldered to the board, instead of prying it out and dropping it into a programmer.
The old workflow was physical: lever the EPROM out of its socket, erase it for fifteen minutes under a UV lamp, burn it in a separate programmer and push it back in. Once flash and EEPROM sat on the same die as the processor, the chip could accept new code through a few pins on the board. Atmel's AVR family did it in the late 90s with four SPI signals plus reset, and Microchip calls the same idea ICSP on its PIC parts.
The consequences were bigger than they sound. Boards could be built with surface-mount parts and no sockets, programmed as the last step on the production line, and updated in the field. It also paved the way for a bootloader that takes over the job via UART or USB — which is how an Arduino gets by without a separate programmer. The legacy lives on as a six-pin header on countless boards whose owners have long forgotten what it is for.