double dagger

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A reference mark (‡) used in printing and writing. Also called diesis.

Wiktionary

1. n. The typographical sign ‡ that is used to indicate a footnote or reference

GNU Webster's 1913

1. n. a mark of reference [‡] which comes next in order after the dagger.
2. adj. (Print.) a reference mark (‡) next to the dagger (†) in order; a diesis.

WordNet 3.0

1. n. a character used in printing to indicate a cross reference or footnote