- sinuous: Curving in and out.
- sinus: An opening or cavity.
- siren: A sea-nymph, described by Homer as dwelling between the island of Circe and Scylla.
- sirocco: hot winds from Africa.
- sisterhood: A body of sisters united by some bond of sympathy or by a religious vow.
- skeptic: One who doubts any statements.
- skepticism: The entertainment of doubt concerning something.
- skiff: Usually, a small light boat propelled by oars.
- skirmish: Desultory fighting between advanced detachments of two armies.
- sleight: A trick or feat so deftly done that the manner of performance escapes observation.
- slight: Of a small importance or significance.
- slothful: Lazy.
- sluggard: A person habitually lazy or idle.
- sociable: Inclined to seek company.
- socialism: A theory of civil polity that aims to secure the reconstruction of society.
- socialist: One who advocates reconstruction of society by collective ownership of land and capital.
- sociology: The philosophical study of society.
- Sol: The sun.
- solace: Comfort in grief, trouble, or calamity.
- solar: Pertaining to the sun.
- solder: A fusible alloy used for joining metallic surfaces or margins.
- soldier: A person engaged in military service.
- solecism: Any violation of established rules or customs.
- solicitor: One who represents a client in court of justice; an attorney.
- solicitude: Uneasiness of mind occasioned by desire, anxiety, or fear.
- soliloquy: A monologue.
- solstice: The time of year when the sun is at its greatest declination.
- soluble: Capable of being dissolved, as in a fluid.
- solvent: Having sufficient funds to pay all debts.
- somber: Gloomy.
- somniferous: Tending to produce sleep.
- somnolence: Oppressive drowsiness.
- somnolent: Sleepy.
- sonata: An instrumental composition.
- sonnet: A poem of fourteen decasyllabic or octosyllabiclines expressing two successive phrases.
- sonorous: Resonant.
- soothsayer: One who claims to have supernatural insight or foresight.
- sophism: A false argument understood to be such by the reasoner himself and intentionally used to deceive
- sophistical: Fallacious.
- sophisticate: To deprive of simplicity of mind or manner.
- sophistry: Reasoning sound in appearance only, especially when designedly deceptive.
- soprano: A woman's or boy's voice of high range.
- sorcery: Witchcraft.
- sordid: Of degraded character or nature.
- souvenir: A token of remembrance.
- sparse: Thinly diffused.
- Spartan: Exceptionally brave; rigorously severe.
- spasmodic: Convulsive.
- specialize: To assume an individual or specific character, or adopt a singular or special course.
- specialty: An employment limited to one particular line of work.