What is Blank Endorsement?

Legal Definition
Blank endorsement of a financial instrument, such as a cheque, is only a signature, not indicating the payee. The effect of this is that it is payable only to the bearer – legally, it transforms an order instrument ("pay to the order of (the payee)") into a bearer instrument ("pay to the bearer"). It is one of the types of endorsement of a negotiable instrument.

It is "an endorsement consisting of nothing but a signature and allowing any party in possession of the endorsed item to execute a claim."

A blank endorsement is a commonly known and accepted term in the legal and business worlds.

This is also called an endorsement in blank or blank endorsement.

The prevalent spelling in American English is endorsement; the minority convention, indorsement, is found in older American documents, although the revised Uniform Commercial Code Article on negotiable instruments retains the older spelling.
-- Wikipedia
Legal Definition
An endorsement of a negotiable instrument that does not name a transferee and that makes the instrument payable to the bearer.
Legal Definition
An endorsement that does not say who the favor is being made to making it a negotiable instrument. Any holder owns it and the property. AKA endorsement in blank.
Legal Definition
Contrad. An endorsement which does not mention the name of the person in whose favor it is made; it is usually made by writing the name of the endorser on the back of the bill. Chit. Bills, 170.

2. When a bill or note has been endorsed in blank, its negotiability cannot afterwards be restrained. 1 Esp. N. P. Cas. 180; 1 Bl. Rep. 295. As many persons as agree may join in suing on a bill when endorsed in blank; for although it was given to one alone, yet by allowing the others to join in the suit, he has 'Made them sharers in his rights. 8 Camp. N. P. Cas. 239. Vide Endorsement; Negotiable paper; Restrictive endorsement.
-- Bouviers Law Dictionary
Legal Definition
One made by the mere writing of the endorser's name on the back of the note or bill, without mention of the name of any person in whose favor the endorsement is made, but with the implied understanding that any lawful holder may fill in his own name above the endorsement if he so chooses. See Thornton v. Moody, 11 Me. 256; Scollans v. Rollins, 179 Mass. 346, 60 N. E. 983, 88 Am. St. Ren. 386; Malone v. Garver, 3 Neb. (Unof.) 710, 92 N. W. 726.
-- Black's Law Dictionary
Legal Definition
The endorsement of a bill of exchange or promissory note, by merely writing the name of the endorsee, without mentioning any person to whom the bill or note is to be paid; called "blank," because a blank or space is left over it for the insertion of the name of the endorsee, or of any subsequent holder. Otherwise called an endorsement "in blank." 3 Kent, Comm. 89; Story, Prom. Notes, § 138.
-- Black's Law Dictionary
Legal Definition
One which does not name an endorsee.
-- Ballentine's Law Dictionary