Bitstream

The file that programs the FPGA.

The bitstream is the binary output of the whole flow — synthesis, place and route, then bitstream generation — containing the configuration of every LUT, routing switch and I/O. Load it over JTAG for development, or from flash for production boot; SRAM-based FPGAs (most of them) forget it at power-off, flash-based parts (MAX 10, some Lattice) keep it.

Bitstream formats are vendor-proprietary, except where the open-source community reverse-engineered them (iCE40 and ECP5 — the basis of the open yosys/nextpnr flow).