A build is reproducible if given the same source code, build environment and build instructions, any party can recreate bit-by-bit identical copies of all specified artifacts.
The relevant attributes of the build environment, the build instructions and the source code as well as the expected reproducible artifacts are defined by the authors or distributors. The artifacts of a build are the parts of the build results that are the desired primary output.Explanations
Source code is usually a checkout from version control at a specific revision or a source code archive.
Relevant attributes of the build environment would usually include dependencies and their versions, build configuration flags and environment variables as far as they are used by the build system (eg. the locale). It is preferable to reduce this set of attributes.
Artifacts would include executables, distribution packages or filesystem images. They would not usually include build logs or similar ancillary outputs.
The reproducibility of artifacts is verified by bit-by-bit comparison. This is usually performed using cryptographically secure hash functions.
Authors or distributors means parties that claim reproducibility of a set of artifacts. These may be upstream authors, distribution maintainers or any other distributor.