| specie | A coin or coins of gold, silver, copper, or other metal. |
| species | A classificatory group of animals or plants subordinate to a genus. |
| specimen | One of a class of persons or things regarded as representative of the class. |
| specious | Plausible. |
| spectator | One who beholds or looks on. |
| specter | Apparition. |
| spectrum | An image formed by rays of light or other radiant energy. |
| speculate | To pursue inquiries and form conjectures. |
| speculator | One who makes an investment that involves a risk of loss, but also a chance of profit. |
| sphericity | The state or condition of being a sphere. |
| spheroid | A body having nearly the form of a sphere. |
| spherometer | An instrument for measuring curvature or radii of spherical surfaces. |
| spinous | Having spines. |
| spinster | A woman who has never been married. |
| spontaneous | Arising from inherent qualities or tendencies without external efficient cause. |
| sprightly | Vivacious. |
| spurious | Not genuine. |
| squabble | To quarrel. |
| squalid | Having a dirty, mean, poverty-stricken appearance. |
| squatter | One who settles on land without permission or right. |
| stagnant | Not flowing: said of water, as in a pool. |
| stagnate | To become dull or inert. |
| stagnation | The condition of not flowing or not changing. |
| stagy | Having a theatrical manner. |
| staid | Of a steady and sober character. |
| stallion | An uncastrated male horse, commonly one kept for breeding. |
| stanchion | A vertical bar, or a pair of bars, used to confine cattle in a stall. |
| stanza | A group of rimed lines, usually forming one of a series of similar divisions in a poem. |
| statecraft | The art of conducting state affairs. |
| static | Pertaining to or designating bodies at rest or forces in equilibrium. |
| statics | The branch of mechanics that treats of the relations that subsist among forces in order. |
| stationary | Not moving. |
| statistician | One who is skilled in collecting and tabulating numerical facts. |
| statuesque | Having the grace, pose, or quietude of a statue. |
| statuette | A figurine. |
| stature | The natural height of an animal body. |
| statute | Any authoritatively declared rule, ordinance, decree, or law. |
| stealth | A concealed manner of acting. |
| stellar | Pertaining to the stars. |
| steppe | One of the extensive plains in Russia and Siberia. |
| sterling | Genuine. |
| stifle | To smother. |
| stigma | A mark of infamy or token of disgrace attaching to a person as the result of evil-doing. |
| stiletto | A small dagger. |
| stimulant | Anything that rouses to activity or to quickened action. |
| stimulate | To rouse to activity or to quickened action. |
| stimulus | Incentive. |
| stingy | Cheap, unwilling to spend money. |
| stipend | A definite amount paid at stated periods in compensation for services or as an allowance. |
| Stoicism | The principles or the practice of the Stoics-being very even tempered in success and failure. |