What is Seal?

Legal Definition
In the law, a seal affixed to a contract or other legal instrument has had special legal significance at various times in the jurisdictions that recognise it. In the courts of common law jurisdictions, a contract which was sealed ("made under seal") was treated differently from other written contracts (which were "made under hand"), although this practice gradually fell out of favour in most of these jurisdictions in the 19th and early 20th century. The legal term seal arises from the wax seal used throughout history for authentication (among other purposes).

Originally, only a wax seal was accepted as a seal by the courts, but by the 19th century many jurisdictions had relaxed the definition to include an impression in the paper on which the instrument was printed, an embossed paper wafer affixed to an instrument, a scroll made with a pen, or the printed words "Seal" or "L.S." (standing for the Latin term locus sigilli meaning "place of the seal").

Notwithstanding their reduced significance, seals are still used on contracts, usually in the impression on paper form.
-- Wikipedia
Legal Definition
A device used to create an impression or imprint on paper utilizing wax or a stamp. The seal is used to execute a legal document or guarantee the document's authenticity.
See also
Legal Definition
An Impression upon wax, wafer or some other tenacious substance capable of being impressed. Allen v. Sullivan R. Co., 32 N. H. 449; Solon v. Williamsburgh Sav. Bank, 114 N. Y. 132, 21 N. E. 168; Alt v. Stoker, 127 Mo. 471, 30 S. W. 132; Bradford v. Randall, 5 Pick. (Mass.) 497; Osborn v. Kistler, 35 Ohio St. 102; Hopewell Tp. v. Amwell Tp., 6 N. J. Law, 175; Jones v. Logwood, 1 Wash. (Vat) 48. A seal is a particular sign, made to attest in the most formal manner, the execution of an instrument. Code Civ. Proc. Cal. ยง 1930. Merlin defines a seal to be a plate of metal with a flat surface, on which is engraved the arms of a prince or nation, or private individual, or other device, with which an impression may be made on wax or other substance on paper or parchment in order to authenticate them. The impression thus made is also called a "seal." R6pert. mot "8ceau."
-- Black's Law Dictionary
Legal Definition
Any mark clearly appearing to have been intended by the person using it to be his seal. See 50 Am. St. Rep. 156, note.
-- Ballentine's Law Dictionary