noun

• (cryptography) A book, table, database, or other object that stores the mapping between plaintext words or phrases and their equivalents in a code.
  "In the 19th century codebooks were used not so much for secrecy as for compression, to bring down the prohibitive cost of telegraph communication. […] But when Dr. Bellovin hunted though the card catalog, his interest was piqued by an 1882 codebook whose title included the word “secrecy.”"
• (computer science) A lookup table.
  "We assume that DWT coefficients belong to a codeblock that is divided by its quantization step size in advance and the two codebooks, C⁰ and C¹, for the two quantizers can be defined as […]"
• (medicine) A list of disorders or conditions and the codes that represent them.
  "The change, which has so far been approved by each committee that has considered it, is under review for the next edition of the W.H.O. codebook, which classifies diseases and influences the treatment of patients worldwide."