What is Arson?

Legal Definition
Arson is the crime of intentionally, deliberately and maliciously setting fire to buildings, wildland areas, dumpsters, vehicles or other property with the intent to cause damage. It may be distinguished from other causes such as spontaneous combustion and natural wildfires. Arson often involves fires deliberately set to the property of another or to one's own property so as to collect insurance compensation.

A person who commits this crime is called an arsonist. More often than not, arsonists use accelerants (such as gasoline or kerosene) to ignite, propel, and directionalize fires.
-- Wikipedia
Legal Definition
A crime at common law, originally defined as the malicious burning of the dwelling of another. Depending upon the jurisdiction, the intentional setting of a fire to a building, or else the intentional setting of a fire to a building where people live.
Arson as a state crime
Most states no longer require that the property damaged or destroyed be a dwelling or even a building, and the knowing burning of personal property without consent or with unlawful intent is generally enough to constitute arson.1 Although arson is generally a felony, many state laws include different degrees of severity depending on the defendant's intent, how the fire or explosion was caused, and whether the fire or explosion resulted in physical injury or death. The penalties associated with a conviction of arson vary widely depending on the degree of arson. For example, the New York Penal Law includes five possible degrees of arson, where the fifth degree is a class A misdemeanor punishable by no more than year in jail, and the first degree is a class A-1 felony carrying a minimum sentence of 15 years and a maximum sentence of 25 years. See NY Penal Law Article 70.
Arson as a federal crime
Under 18 U.S.C. § 844(i), it is a federal crime to damage or destroy, "by means of fire or an explosive, any . . . property used in interstate or foreign commerce or in any activity affecting interstate or foreign commerce." Any person found guilty of arson under this statute may be sentenced to a maximum of 20 years in jail, with a minimum of 5 years. If a violation resulted in personal injury to any person, the maximum sentence is 40 years in jail, with a minimum of 7 years. In Jones v. U.S., 529 U.S. 848 (2000), the Supreme Court held that arson of an owner-occupied private residence not used for any commercial purpose is not subject to federal prosecution under § 844(i), which covers only property used in an activity affecting commerce.
See also
Legal Definition
Criminal law. At common law an offence of the degree of felony; and is defined by Lord Coke to be the malicious and voluntary burning of the house of another, by night or day. 3 Inst. 66.

2. In order to make this crime complete, there must be, 1st, a burning of the house, or some part of it; it is sufficient if any part be consumed, however small it may be. 9 C. & P. 45; 38 E. C. L. R. 29; 16 Mass. 105. 2d. The house burnt must; belong to another; but if a man set fire to his own house with a view to burn his neighbor's, and does so, it is at least a great misdemeanor, if not a felony. 1 Hale, P. C. 568; 2 East, P. C. 1027; 2 Russ. 487. 3d. The burning must have been both malicious and willful.

3. The offence of arson at common law, does not extend further than the burning of the house of another. By statute this crime is greatly enlarged in some of the states, as in Pennsylvania, where it is extended to the burning of any barn or outhouse having bay or grain therein; any barrack, rick or stack of hay, grain, or bark; any public buildings, church or meeting-house, college, school or library. Act 23d April, 1829; 2 Russell on Crimes, 486; 1 Hawk. P. C. c. 39 4 Bl. Com. 220; 2 East, P. C. c. 21, s. 1, p. 1015; 16 John. R. 203; 16 Mass. 105. As to the extension of the offence by the laws of the United States, see Stat. 1825, c. 276, 3 Story's L. U. S. 1999.
-- Bouviers Law Dictionary
Legal Definition
Arson, at common law, is the act of unlawfully and maliciously burning the house of another man. 4 Steph. Comm. 99; 2 Russ. Crimes, 896; Steph. Crim. Dig. 298.

Arson, by the common law, is the willful and malicious burning of the house of another. The word "house," as here understood, includes not merely the dwelling-house, but all outhouses which are parcel thereof. State v. McGowan, 20 Conn. 245, 52 Am. Dec. 336; Graham v. State, 40 Ala. 664; Allen v. State, 10 Ohio St. 300; State v. Porter, 90 N. C. 719; Hill v. Com., 98 Pa. 195; State v. McCoy, 162 Mo. 383, 62 S. W. 991.

Arson is the malicious and willful burning of the house or outhouse of another. Code Ga. 1882, § 4375. Arson is the willful and malicious burning of a building with intent to destroy it. Pen. Code Cal. § 447.

Degrees of arson. In several states, this crime is divided into arson in the first, second, and third degrees, the first degree including the burning of an inhabited dwelling-house in the night-time; the second degree, the burning (at night) of a building other than a dwelling-house, but so situated with inference to a dwelling-house as to endanger it; the third degree, the burning of any building or structure not the subject of arson in the first or second degree, or the burning of property, his own or another's, with intent to defraud or prejudice an insurer thereof. People v. Durkin, 6 Parker, Cr. R. (N. Y.) 248; People v. Fanshawe, 65 Hun, 77, 19 N. Y. Supp. 865; State v. McCoy, 162 Mo. 383, 62 S. W. 991; State v. Jessup, 42 Kan. 422, 22 Pac. 627.
-- Black's Law Dictionary
Legal Definition
The malicious burning of the dwelling-house of another. See 71 Am. St. Rep. 266.
-- Ballentine's Law Dictionary