A failure to take the care a reasonable person would have taken in the same situation, which causes harm to someone else.
Negligence has three parts: the other party owed the injured person a duty of care, they breached that duty by failing to take reasonable precautions, and the breach caused the injury. All three must be shown.
In Queensland and New South Wales the test is set out in civil liability legislation, which also lists matters the court weighs, such as how likely the harm was, how serious it could be, and how burdensome the precautions would have been.