Bitstream
Beginner
The file that sets up an FPGA. It holds a 1 or 0 for every table and switch.
Novice
The configuration file for an FPGA: every LUT bit, switch setting and block mode, plus commands for loading them. Loading a different bitstream turns the same chip into a different circuit.
Expert
A command stream that writes configuration frames (on AMD 7 series, 101 32-bit words each) and block-RAM contents, usually with CRC checks and often encryption. Formats are vendor-specific and mostly undocumented.
Explained in Lookup tables and the FPGA fabric (Architectures).
See also: Configuration memory, Partial reconfiguration.