Scabbers (PS ch. 6): Ron's rat, inherited from his brother Percy.
Scamander, Newt (PS ch. 5): Author of Fantastic Beasts and Where to Find Them.
Etym: The name of a river mentioned by Homer, used in the late 19th century to mean "to wander about" or "to take a devious course".
Seamus Finnigan: Etym: The Irish form of James.
Seeker: The player on a Quidditch team who chases the Golden Snitch. Catching the Snitch scores 150 points and ends the match.
Severus Snape: Etym: The name of several Roman emperors, most notably Septimius Severus, who shifted the imperial power base from the nobility to the army. The word is Latin for "harsh, severe, strictly correct". Also, cognate to the later name Severian.
One Latin dictionary-- but just one out of 5 or 6 I've consulted by now-- suggests that the word may be from se verus, "one who separates the truth (from falsehood)".
Sickle: 1/17th of a Galleon, or 29 Knuts.
Etym: "Silver sickle" is a popular poetic term for the crescent moon.
Sirius Black: Etym: Canis Major, the Dog Star.
Slytherin House: One of the four houses of Hogwarts. Its defining virtues are unclear: Hagrid believes it selects for evil; Draco Malfoy thinks it's where the "right people" go; the Sorting Hat just selects for sneaky cleverness. The head of the house is Professor Snape. Badge: Vert, a snake erect argent. The house ghost is the Bloody Baron.
Etym: Invented; undoubtedly meant to sound like "slithering".
Smeltings: The school that Vernon Dursley attended and which Dudley now goes to.
Etym: Probably invented from smelting.
Snape, Severus: The Potions teacher at Hogwarts, and head of Slytherin House. Has a reputation for being mean and a particular dislike of Harry, but hey, if your childhood antagonist had grown up to be a famous martyr, and then his kid was the most famous wizard in existence at the age of eleven, you'd be a bit crabby too.
Etym: Stated by Rowling on numerous occasions to be from a village name. The name comes from Old Norse snap or Old English snoep, meaning "poor grazing" or "winter pasture", and is still used in Sussex as a term for ground that is too boggy to cultivate. Not too far from the meaning of Nettleship.
Snowy (PS ch. 2): One of Mrs. Figg's cats.
Sorceror's Stone: Name for the Philosopher's Stone in US editions. The US editor felt the word "philosopher" would scare people away from buying the book.
Sorting Ceremony: How new students at Hogwarts are assigned to a house. They put on the Sorting Hat, which decides where to send them.
Sorting Hat: A sentient magical artifact which looks into the minds of new students to decide which house they should be in.
Spinnet, Alicia (PS ch. 11): A Chaser for the Gryffindor Quidditch team.
Etym: May be derived from a name used to mean a thorn bush or the crest of a hill. Also, a spinet is an instrument resembling a harpsichord.
Spore, Phyllida (PS ch. 5): Author of One Thousand Magical Herbs and Fungi.
Etym: Meant to mean what it looks like.
Sprout: The witch who teaches Herbology at Hogwarts.
Etym: Derived from an Old English name possibly also meaning "sprout".
squid, giant: A denizen of the lake adjoining Hogwarts, something along these lines.
Standard Book of Spells, The: Name of a series of basic spell textbooks used at Hogwarts.
Stonewall: The name of the local comprehensive school (what in the US would be a public combined high school and junior high) Harry was going to be sent to before he knew about Hogwarts.
Study of Recent Developments in Wizardry, A: A book in the Hogwarts library.
Supreme Mugwump: One of Dumbledore's titles.
Surrey: The area in which Little Whinging is located. Surrey is fairly thick with businessmen who commute to London.
Susan Bones: Etym: From the the Hebrew Shushannah, "lily", the name of the heroine of the Book of Susannah, an apocryphal addition to the Book of Daniel.
Switch, Emeric (PS ch. 5): Author of A Beginner's Guide to Transformation.
Etym: What it looks like.
Switching Spells: Spells used in Transfiguration.