The Collaborative International Dictionary of English v.0.48:

Bookplate \Book"plate`\, n.
   A label, placed upon or in a book, showing its ownership or
   its position in a library.
   [1913 Webster]

WordNet (r) 3.0 (2006):

bookplate
    n 1: a label identifying the owner of a book in which it is
         pasted [syn: bookplate, ex libris]