| Term | Definition |
| Nefarious | intensely wicked or vicious |
| Neonate | a newborn child |
| Nihilism | the belief that traditional values and beliefs are unfounded and that existences is useless; the belief that conditions in the social organization are so bad as to make destruction desirable |
| Nomenclature | a system of scientific names |
| Non sesquitur | a statement that does not follow logically from anything previously said |
| Novel | new and not resembling anything formerly known |
| Obdurate | stubbornly persistent, resistant to persuasion |
| Obfuscate | to confuse, make obscure |
| Obstinate | unreasonably persistent |
| Olfactory | relating to the sense of smell |
| Oligarchy | a government in which a small group exercises supreme control |
| Onus | a burden, an obligation |
| Opine | to express an opinion |
| Opportunist | one who takes advantage of any opportunity to achieve an end, with little regard for principles |
| Opprobrious | disgraceful, shameful |