What is Ballot?

Legal Definition
A ballot is a device used to cast votes in an election, and may be a piece of paper or a small ball used in secret voting. It was originally a small ball (see blackballing) used to record decisions made by voters.

Each voter uses one ballot, and ballots are not shared. In the simplest elections, a ballot may be a simple scrap of paper on which each voter writes in the name of a candidate, but governmental elections use preprinted ballots to protect the secrecy of the votes. The voter casts his/her ballot in a box at a polling station.

In British English, this is usually called a "ballot paper". The word ballot is used for an election process within an organisation (such as a trade union "holding a ballot" of its members).
-- Wikipedia
Legal Definition
Government. A diminutive ball, i. e. a little ball used in giving votes; the act itself of giving votes. A little ball or ticket used in voting privately, and, for that purpose, put, into a box, (commonly called a ballot-box,) or into some other contrivance.
-- Bouviers Law Dictionary
Legal Definition
In the law of elections. A slip of paper bearing the names of the offices to be filled at the particular election and the names of the candidates for whom the elector desires to vote; it may be printed, or written or partly printed and partly written, and is deposited by the voter in a "ballot-box" which is in the custody of the officers holding the election. Opinion of Justices, 19 RI. 729, 36 Atl. 716, 36 L R. A. 547; Bris-bin v. Cleary, 26 Minn. 107, 1 N. W. 825; State v. Timothy, 147 Mo. 532, 49 S. W. 500; Taylor v. Bleakley, 55 Kan. 1, 39 Pac. 1045, 28 L. R. A. 683, 49 Am. St. Rep. 233.

Also the act of voting by balls or tickets. A ballot is a ticket folded in such a manner that nothing written or printed thereon can be seen. Pol. Code Cal. § 1186.

A ballot is defined to be "a paper ticket containing the names of the persons for whom the elector intends to vote and designating the office to which each person so named is intended by him to be chosen." Thus a ballot, or a ticket, is a single piece of paper containing the names of the candidates and the offices for which they are running. If the elector were to write the names of the candidates upon his ticket twice or three or more times, he does not thereby make it more than one ticket. People v. Holden, 28 Cal. 136. See Joint ballot.
-- Black's Law Dictionary
Legal Definition
A ticket or paper used in voting; to vote at an election. See 5 Ann. Cas. 861.
-- Ballentine's Law Dictionary