What is Police Power?

Legal Definition
In United States constitutional law, police power is the capacity of the states to regulate behavior and enforce order within their territory for the betterment of the health, safety, morals, and general welfare of their inhabitants. Under the Tenth Amendment to the United States Constitution, the powers not delegated to the Federal Government are reserved to the states or to the people. This implies that the Federal Government does not possess all possible powers, because most of these are reserved to the State governments, and others are reserved to the people.

Police power is exercised by the legislative and executive branches of the various states through the enactment and enforcement of laws. States have the power to compel obedience to these laws through whatever measures they see fit, provided these measures do not infringe upon any of the rights protected by the United States Constitution or in the various state constitutions, and are not unreasonably arbitrary or oppressive. Methods of enforcement can include legal sanctions, physical means, and other forms of coercion and inducement. Controversies over the exercise of state police power can arise when exercise by state authorities conflicts with individual rights and freedoms.
-- Wikipedia
Legal Definition
The powers granted by the constitution to the state in order to govern, establish, adopt as well as enforce laws that are designed for the protection as well as preservation of the public health. The government also gets the right to make use of private property for public usage.
Legal Definition
The power vested in a state to establish laws and ordinances for the regulation and enforcement of its police as above defined. The power vested in the legislafure to make, ordain, and establish nil manner of wholesome and reasonable laws, statutes, and ordinances, either with penalties or without, not repugnant to the constitution, as they shall judge to be for the good and welfare of the commonwealth, and of the subj'ects of the same. Com. v. Alger, 7 Cash. (Mass.) 85. The police power of the state is an authority conferred by the American constitutional system upon the individual slates, through which they are enabled to establish a special department of police; adopt such regulations as tend to prevent the commission of fraud, violence, or other offenses against the state ; aid in the arrest of criminals; and secure generally the comfort, health, and prosperity of the state, by preserving the public order, preventing a conflict of rights in the common intercourse of the citizens, and insuring to each an uninterrupted enjoyment of nil the privileges conferred upon him by the laws of his country. Lalor, Pol. Enc. s. v. It is true that the legislation which secures to nil protection in their rights, and the equal use and enjoyment of their property, embraces an almost infinite variety of subjects. Whatever affects the peace, good order, morals, and health of the community comes within its scope; and every one must use and enjoy his property subject to the restrictions which such legislation imposes. What is termed the "police power" of the state, which, from the language often used respecting it, one would suppose to be an undefined and irresponsible element in government, can only interfere with the conduct of individuals in their intercourse with each other, and in the use of their property, so far as may be required to secure these objects. Munn v. Illinois, 94 U. Sl 145, 24 L. Ed. 77. For other definitions, see Slaughterhouse Cases, 16 Wall. 62, 2i L. Ed. 394; Stone v. Mississippi, 107 IJ. S. 818, 25 L. Ed. 1079; Thorpe v. Rutland & B. R. Co., 27 Vt. 140, 62 Am. Dec. 625; People v. Steele, 231 111. 340, 83 N. E. 236, 14 In R. A. (N. S.) 361, 121 Am. St Rep. 321; Dreyfus v. Boone, 88 Ark. 353, 114 S. W. 718; Carpenter v. Reliance Realty Co., 103 Mo. App. 480,77 S. W. 1004; State v, Dalton, 22 R. I. 77, 46 Atl. 234, 48 L. It. A. 775, 84 Am. St Rep. 818; Deems v. Baltimore, 80 Md. 164, 30 Atl. 648, 26 L. In A. 541, 45 Am. St. Rep. 339; In re Clark, 65 Conn. 17, 31 Atl. 522, 28 L. R. A. 242; Mathews v. Board of Education, 127 Mich. 530, 86 N. W. 1036, 54 L. R. A. 736.
-- Black's Law Dictionary
Legal Definition
That authority which resides in every sovereignty to pass all laws for the internal regulation and government of the state necessary for the public welfare. See 53 Am. St. Rep. 572, note. See, also, 104 Am. St. Rep. 638, note.
-- Ballentine's Law Dictionary