If, building the same source code, on the same operating system, using the same compiler and build instructions, anybody is able to reproduce a bit-by-bit identical copy of an artifact, then it means that the builds are reproducible. The GNU project has been involved with reproducible builds since 1990, when they were making active efforts to make the builds for their GNU coreutils bit-for-bit reproducible. The Debian community was so interested in what had been the output of that discussion that a wiki page about reproducible builds was created, with the idea to get five packages to build reproducibly as a proof-of-concept.