What is negligence in law?

Art. 1173. The fault or negligence of the obligor consists in the omission of that diligence which is required by the nature of the obligation and corresponds with the circumstances of the persons, of the time and of the place. When negligence shows bad faith, the provisions of Articles 1171 and 2201, paragraph 2, shall apply.

If the law or contract does not state the diligence which is to be observed in the performance, that which is expected of a good father of a family shall be required. (Civil Code of the Philippines)

Law or contract may require a different level of care. For example, parties may stipulate that no care (zero level of care) is required (which almost never happens). An example of a higher level of care required by law is that required from common carriers; they are expected to comply with their obligation to deliver persons or goods from one place to another with extraordinary diligence.