pocket edition; pocket-sized book
Refers to a small-format book designed to be carried in a pocket. The term is somewhat formal and may be encountered in bibliographic contexts.
This dictionary is a pocket edition, so it's convenient to carry around.
Compound of 掌中 (しょうちゅう, 'in the palm') + 本 (ほん, 'book'), literally 'palm-sized book'.