The Washington Pattern Jury instructions do a pretty good job of defining negligence in plain language. These are the instructions a judge would typically give to a jury. Here it is:
“Negligence is the failure to exercise ordinary care. It is the doing of some act that a reasonably careful person would not do under the same or similar circumstances or the failure to do some act that a reasonably careful person would have done under the same or similar circumstances.”
Read more about negligence here.