Wiktionary
n. (context book printing English) a page before the title page containing the book's title but no title-page details
WordNet
n. a first page of some books displaying only the title of the book [syn: bastard title]
Wikipedia
The half title or bastard title is a page carrying nothing but the title of a book—as opposed to the title page, which also lists subtitle, author, publisher and edition. The half title is usually counted as the very first page (p. i) in a printed book.
The half title can have some ornamentation of the book's title, or it can be plain text.
Theodore Low De Vinne distinguishes between half title and bastard title in his series The Practice of Typography, saying:
The half-title should not be confounded with the bastard title. The half-title follows the title and begins the first page of text; the bastard title, usually a single line in capital letters, precedes the full title, and takes a separate leaf with blank verso.