| notched weir | ฝายสำหรับวัด (น้ำ) |
| nut | แป้นเกลียว |
| Normal strain: | (english) Strain measuring the intensity of deformation along an axis. Normal strain is usually denoted by . Average normal strain between two points is calculated as (Delta L / L), where L is the original distance between the points, and L is the change in that distance. Normal strain is often simply called strain. |
| Normal stress: | (english) Stress acting perpendicular to an imaginary plane cutting through an object. Normal stress has two senses: compression and tension. Normal stress is often simply called stress. |
| Nitrification | (Environmental Engineering) The biological oxidation of ammonia and ammonium sequentially to nitrite and then nitrate. It occurs naturally in surface waters, and can be engineered in wastewater treatment systems. The purpose of nitrification in wastewater treatment systems is a reduction in the oxygen demand resulting from the ammonia. |
| Nitrogen fixation | (Environmental Engineering) The conversion of atmospheric (or dissolved) nitrogen gas into nitrate by microorganisms. |
| Nitrogenous oxygen demand (NOD) | (Environmental Engineering) The amount of oxygen required to oxidize any ammonia present in a water. |
| Nonpoint source pollution (NPSP) | (Environmental Engineering) Any pollution from a source which cannot be attributed to a particular discharge point, e.g. from agricultural crops, city streets, construction sites, etc. |
| NPDES | (Environmental Engineering) The National Pollutant Discharge Elimination System. The discharge criteria and permitting system established by the U.S. EPA as a result of the Clean Water Act and its subsequent amendments or the permit required by each discharger as a result of the Clean Water Act. |
| Navigation analysis - | (Software Engineering) a Web engineering action that establishes how a user will navigate between various elements (e.g., content, functions) of a WebApp |