PEEK and POKE
The BASIC commands that read and wrote a single memory address — and so the home computer's only way around whatever BASIC itself couldn't do.
PEEK(address) returns a byte; POKE address,value writes one. On machines with memory-mapped video and sound chips that made them a universal tool: POKE 53280,0 turned the C64's border black, and since Commodore BASIC V2 had no commands for sound or graphics at all, half the manual was address tables. SYS 64738, a jump into the KERNAL's reset routine, was the same idea in machine code.
Magazine type-in listings were often mostly DATA lines that a loop POKEd into memory. And "pokes" became a thing among gamers: magic addresses for infinite lives, printed on readers' pages long before every disk shipped with a trainer.