noun
only extant copy; sole surviving copy
Used for a book or manuscript of which no other copies are known to exist. Often encountered in bibliographic or antiquarian contexts.
この図書館には、世界で唯一の孤本が所蔵されている。
This library holds the only extant copy in the world.
写本 is a manuscript copy, which may exist in multiple copies, while 孤本 is the sole surviving copy.
稀覯本 is a rare book, not necessarily unique; 孤本 is specifically the only known copy.
Compound of 孤 (ko, 'alone, solitary') and 本 (hon, 'book'). The exact historical derivation is uncertain, but the term is conventionally used in bibliographic contexts.