Real Dictionary

dog-ear in American English
noun: a corner of a page folded over like a dog's ear, as by careless use, or to mark a place
transitive verb: to fold down the corner of (a page in a book)

dog-ear in British English
a turned-down corner of the leaf of a book
to turn down the corner of (a leaf or leaves in a book)
noun: a folded-down corner of a page
verb: to fold down the corner of (a page)