Raw machine code. When computers were first "programmed" from an input device, rather than by being rewired, they were fed input in the form of numbers, which they then interpreted as commands. This was really low level, and a program fragment might look like "010307 010307". Almost no one programs in machine language anymore, because translators are nearly trivial to write.
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Nearby terms: First Fit « first generation « first generation computer « first generation language » first-in first-out » first normal form » first-order