Negligence is one of three prongs of tort law (the two others being strict liability and intentional wrongs, but more on these in future posts). Negligence is defined by Black's Law Dictionary (Second Ed.) as "The failure to exercise the standard of care that a reasonably prudent person would have exercised in a similar situation; any conduct that falls below the legal standard established to protect others against unreasonable risk of harm." But what does that really mean?
The word refers to conduct that is considered objectively unreasonable. For example, if you slipped and fell on ice in front of a store, and the store-owner had notice that the gutter above his store was leaking water onto the sidewalk and that this water would freeze in the winter, then that store-owner may have acted unreasonably, or negligently, in failing to correct the problem. It was foreseeable – on the part of the store-owner – that someone would slip and fall on the ice (or maybe he or she even knew of slips and falls in the past and did nothing to correct it).
Or, let's take the example of an automobile accident. If a driver is driving erratically and ends up injuring someone, that driver may have been operating his automobile negligently, because he or she could have reasonably foreseen that someone would be injured as a result of his or her unreasonable conduct.