Negligence is 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, except for conduct that is intentionally, wantonly, or willfully disregardful of others’ rights.
The term denotes culpable carelessness.
A tort grounded in this failure, usually expressed in terms of the following elements: duty, breach of duty, causation, and damages.

In Plain English:

If a person is negligent, it basically means that he or she wasn’t as careful as most reasonable people would be in any given situation. For example, if a driver does not check his mirrors or blind spot before making a lane change, that driver was engaging in negligent behavior because most would agree that a reasonable person would check for other cars before making a lane change.

This behavior becomes the tort of negligence when the negligent behavior causes damages to someone else or someone else’s property.