What is Blasphemy?

Legal Definition
Blasphemy is the act of insulting or showing contempt or lack of reverence to a deity, to religious or holy persons or sacred things, or toward something considered sacred or inviolable.

Some religions consider blasphemy as a religious crime. As of 2012, anti-blasphemy laws existed in 32 countries, while 87 nations had hate speech laws that covered defamation of religion and public expression of hate against a religious group. Anti-blasphemy laws are particularly common in Muslim-majority nations, such as those in the Middle East and North Africa, although they are also present in some Asian and European countries.
-- Wikipedia
Legal Definition
Crim. law. To attribute to God that which is contrary to his nature, and does not belong to him, and to deny what does or it is a false reflection uttered with a malicious, design of reviling God. Elym's Pref. to vol. 8, St. Tr.

2. This offence has been enlarged in Pennsylvania, and perhaps most of the states, by statutory provision. Vide Christianity; 11 Serg. & Rawle, 394. In England all blasphemies against God, the Christian religion, the Holy Scriptures, and malicious revilings of the established church, are punishable by indictment. 1 East, P. C. 3; 1 Russ. on Cr. 217.

3. In France, before the 25th of September, 1791, it was a blasphemy also to speak against the holy virgin and the saints, to deny one's faith, to speak with impiety of holy things, and to swear by things sacred. Merl. Rep. h. t. The law relating to blasphemy in that country was totally repealed by the code of 25th of September, 1791, and its present penal code, art. 262, enacts, that any person who, by words or gestures, shall commit any outrage upon objects of public worship, in the places designed or actually employed for the performance of its rites, or shall assault or insult the ministers of such worship in the exercise of their functions, shall be fined from sixteen to five hundred francs, and be imprisoned for a period not less than fifteen days nor more than six months.

4. The civil law forbad the crime of blasphemy; such, for example, as to swear by the hair or the head of God; and it punished its violation with death. Si enim contra homines factae blasphemiae impunitae non relinquuntur; multo magis qui ipsum Deum Blasphemant, digni sunt supplicia sustinere. Nov. 77, ch. 1, §1.

5. In Spain it is blasphemy not only to speak against God and his government, but to utter injuries against the Virgin Mary and the saints. Senen Villanova Y Manes, Materia Criminal, forense, Observ. 11, cap. 3, n
-- Bouviers Law Dictionary
Legal Definition
In English law. Blasphemy is the offence of speaking matter relating to God, Jesus Christ, the Bible or the Book of Common Prayer, intended to wound the feelings of mankind or to excite contempt and hatred against the church by law established or to promote immorality. Sweet.

In American law. Any oral or written reproach maliciously cast upon God, His name, attributes, or religion. Co.m. v. Knee-land, 20 Pick. (Mass.) 213; Young v. State, 10 Lea (Tenn.) 165; Com. v. Spratt, 14 Phila. (Pat) 365; People v. Buggies, 8 Johns. (N. Y.) 290, 5 Am. Dec. 335; Updegraph v.om., 11 Serg. & R. (Pa.) 406 ; 2 Bish. Cr. Law, § 76; Pen. Code Dak. § 31.

In general, blasphemy may be described as consisting in speaking evil of the Deity with an impious purpose to derogate from the divine majesty, and to alienate the minds of others from the love and reverence of God. It is purposely using words concerning God calculated and designed to impair and destroy the reverence, respect, and confidence due to Him as the intelligent creator, governor, and judge of the world. It embraces the idea of detraction, when used towards the Supreme Being, as "calumny" usually carries the same idea when applied to an individual. It is a willful and malicious attempt to lessen men's reverence of God by deny ing His existence, or His attributes as an intelligent creator, governor, and judge of men, and to prevent their having confidence m Him as such. Com. v. Kneeland, 20 Pick. (Mass.) 211, 212.

The use of this word is, in modern law exclusively confined to sacred subjects; but blasphemia and blasphemare were anciently used to signify the reviling by one person of another. Nov. 77,t c. 1, § 1; Spelman.
-- Black's Law Dictionary
Legal Definition
Contumelious reproaches of God. See 22 L. R. A. 353.
-- Ballentine's Law Dictionary