What is Negligence?

Legal Definition
Negligence (Lat. negligentia, from neglegere, to neglect, literally "not to pick up something") is a failure to exercise the appropriate and or ethical ruled care expected to be exercised amongst specified circumstances. The area of tort law known as negligence involves harm caused by failing to act as a form or multiple forms of carelessness, .. Although, not always identified as (necessarily) intentional harm.

According to Jay M. Feinman of the Rutgers University School of Law:

"The core idea of negligence is that people should exercise reasonable care when they act by taking account of the potential harm that they might foreseeably cause to other people."

Someone who suffers loss caused by another's negligence may be able to sue for damages to compensate for their harm. Such loss may include physical injury, harm to property, psychiatric illness, or economic loss. The law on negligence – when one person can sue another person in negligence – is not possible to define in general terms. It can depend on the kind of loss, the relationship between the plaintiff and the defendant, and other factors.
-- Wikipedia
Legal Definition
A failure to behave with the level of care that someone of ordinary prudence would have exercised under the same circumstances. The behavior usually consists of actions, but can also consist of omissions when there is some duty to act (e.g., a duty to help victims of one's previous conduct).
Primary factors to consider in ascertaining whether the person's conduct lacks reasonable care are the foreseeable likelihood that the person's conduct will result in harm, the foreseeable severity of any harm that may ensue, and the burden of precautions to eliminate or reduce the risk of harm. See Restatement (Third) of Torts: Liability for Physical Harm § 3 (P.F.D. No. 1, 2005). Negligent conduct may consist of either an act, or an omission to act when there is a duty to do so. See Restatement (Second) of Torts § 282 (1965).

Five elements are required to establish a prima facie case of negligence: the existence of a legal duty to exercise reasonable care; a failure to exercise reasonable care; cause in fact of physical harm by the negligent conduct; physical harm in the form of actual damages; and proximate cause, a showing that the harm is within the scope of liability.

See also:
Legal Definition
Contracts, torts. When considered in relation, to contracts, negligence may be divided into various degrees, namely, ordinary, less than ordinary, more than ordinary. 1 Miles' Rep. 40.

2. Ordinary negligence is the want of ordinary diligence; slight or less than ordinary negligence, is, the want of great diligence; and gross or more than ordinary negligence, is the want of slight diligence.

3. Three great principles of responsibility, seem naturally to follow this division.

4. - 1. In those contracts which are made for the sole benefit of the creditor, the debtor is responsible only for gross negligence, good faith alone being required of him; as in tile case of a depositary, who is a bailee without reward; Story, Bailm. 62; Dane's Ab. c. 17, a, 2; 14 Serg. & Rawle, 275; but to this general rule, Pothier makes two exceptions. The first, in relation to the contract of a mandate, and the second, to the quasi contract negotiorum gestorum; in these cases, he says, the party undertaking to perform these engagements, is bound to use necessary care. Observation Generale, printed at the end of the Traite des Obligations.

5. - 2. In those contracts which are for the reciprocal benefit of both parties, such as those of sale, of hiring, of pledge, and the like, the party is bound to take, for the object of the contract, that care which a prudent man ordinarily takes of his affairs, and he will therefore be held responsible for ordinary neglect. Jones' Bailment, 10, 119; 2 Lord Raym. 909; Story, Bailm. 23; Pothier, Obs. Gener. ubi supra.

6. - 3. In those contracts made for the sole interest of the party who has received, and is to return the thing which is the object of the contract, such, for example, as loan for use, or commodatum, the slightest negligence will make him responsible. Jones' Bailm. 64, 65; Story's Bailm. 237; Pothier, Obs. Gen. ubi supra.

7. In general, a party who has caused an injury or loss to another in consequence of his negligence, is responsible for all the consequence. Hob. 134; 3 Wils. 126; 1 Chit. TI. 129, 130; 2 Hen. & Munf. 423; 1 Str. 596; 3 East, R. 596. An example of this kind may be found in the case of a person who drives his carriage during a dark night on the wrong side of the road, by which he commits an injury to another. 3 East, R. 593; 1 Campb. R. 497; 2 Cam b. 466; 2 New Rep. 119. Vide Gale and Whatley on Easements, Index, h. t.; 6 T. R. 659; 1 East, R. 106; 4 B. & A; 590; S. C. 6 E. C. L. R. 628; 1 Taunt. 568; 2 Stark. R. 272; 2 Bing. R. 170; 5 Esp. R. 35, 263; 5 B. & C. 550. Whether the incautious conduct of the plaintiff will excuse the negligence of the defendant, see 1 Q. B. 29; 4 P. & D. 642; 3 M. Lyr. & Sc. 9; Fault.

8. When the law imposes a duty on an officer, whether it be by common law or statute, and he neglects to perform it, he may be indicted for such neglect; 1 Salk. R. 380; 6 Mod, R. 96; and in some cases such neglect will amount to a forfeiture of the office. 4 Bl. Com. 140. See Bouv. Inst. Index, h. t.
-- Bouviers Law Dictionary
Legal Definition
The omission to do semething which a reasonable man, guided by those considerations which ordinarily regulate the conduct of human affairs, would do or doing something which a prudent and reasonable man would not do. It must be determined in all cases by reference to the situation and knowledge of the parties and all the attendant circumstances. NitroGlycerin Case, 15 Wall. 536, 21 L. Ed. 206; Blythe v. Birmingham Waterworks Co., 11 Exch. 784. Negligence, in its civil relation, is such an inadvertent imperfection, by a responsible human agent, in the dlscharge of a legal duty, as immediately produces, in an ordinary and natural sequence, a damage to another. Whart. Neg. § 3. It is conceded by all the authorities that the standard by which to determine whether a person has been guilty of negligence is the conduct of the prudent or careful or diligent man. Bigelow, Torte, 261. The failure to observe, for the protection of the interests of another person, that degree of care, precaution, and vigilance which the circumstances justly demand, whereby such other person suffers injury. Co.oley, Torts, 630. The failure to do what a reasonable and prudent person would ordinarily have done under the circumstances of the situation, or the doing what such a person under the existing circumstances would not have done. Baltimore & P. R. Co. v. Jones, 95 U. S. 441, 24 L. Ed. 506. The opposite of care and prudence; the omission to use the means reasonably necessary to avoid injury to others Great Western R. Co. v. Haworth, 39 111. 353. Negligence or carelessness signifies want of care, caution, attention, diligence, or discretion in one having no positive intention to injure the person complaining thereof. The words "reckless," "indifferent," "careless," and "wanton" are never understood to signify positive will or intention, unless when joined with other words which show that they are to receive an artificial or unusual, if not an unnatural, interpretation. Lexington v. Lewis, 10 Bush (Ky.) 077. Negligence is any culpable omission of a positive duty. It differs from heedlessness, in that heedlessness is the doing of an act in violation of a negative duty, without adverting to its possible consequences. In both cases there is inadvertence, and there is breach of duty. Aust. Jur. § 630.
See also
-- Black's Law Dictionary
Legal Definition
The omission to do something which a reasonable man would do under the circumstances, or the doing of something which a prudent and reasonable man would not do. See 48 Neb. 563, 58 Am. St. Rep. 709, 33 L. R. A. 598, 67 N. W. 479.
-- Ballentine's Law Dictionary