Lossless compression
Compression where the original can be reconstructed bit for bit. The opposite of lossy formats like JPEG and MP3, which discard information the eye or ear wouldn't notice anyway.
It rests on two families of technique that are almost always combined: dictionary coding, which replaces repetitions with backward references (LZ77, LZ78, LZW), and entropy coding, which gives common symbols shorter codes (Huffman, arithmetic coding, ANS). Deflate in ZIP and gzip is LZ77 plus Huffman; modern Zstandard and Brotli are the same idea with better dictionaries and ANS. The compression ratio is bounded by the data's entropy — which is why already-compressed material doesn't shrink at all, and why a universal "compress anything by half" cannot exist. Mandatory for code, text, archives and medical imaging. FLAC, PNG and ALAC are lossless; WebP and HEIC have both modes.