Salazar Slytherin: Etym: A name of Basque origin from the Romance sala "hall" and Basque zahar "old". Hmm...
Sally-Anne Perks: Etym: Sally is an alternate form of Sarah, Hebrew for "princess"; Anne is another form of Hannah.
Scabbers: Ron's pet rat for his first three years of school, until the rat turned out to be Peter Pettigrew in disguise.
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".
Scintillation Solution: No idea. It's mentioned by the Kwikspell ad.
Scops owl: A type of owl native to sub-Saharan Africa.
Seamus Finnigan: Etym: The Irish form of James.
Secret-Keeper: The person used in a Fidelius Charm.
Seeker: The player on a Quidditch team who chases the Golden Snitch. Catching the Snitch scores 150 points and ends the match.
Serpensortia: An incantation that conjures a giant snake.
Etym: Latin serpens "snake" + something akin to French sortir "escape, go out" (a cognate to English sortie).
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)".
Shooting Star: A brand of broom.
Shrieking Shack: A building in Hogsmeade reputed to be the most haunted structure in Britain; never actually haunted except by Remus Lupin undergoing werewolf transformations during his school years.
Shrinking Solution: A potion which appears to have the ability to age an animal backwards.
shrivelfig: I have no idea. Invented?
Shunpike, Stan: Etym: In early 20th-century US usage, "to drive along minor roads, avoiding the toll on turnpikes, or for pleasure". This may not be the direct source for Rowling's usage, but probably a similar meaning is intended.
Sibyll Trelawney: Etym: A sibyl is a female oracle or fortuneteller.
Sickle: 1/17th of a Galleon, or 29 Knuts.
Etym: "Silver sickle" is a popular poetic term for the crescent moon.
Silver Arrow: A brand of flying broom, now out of production.
Etym: This was a nickname for Mercedes-Benz race cars up to 1955, when Mercedes-Benz pulled out of racing for a while due to one of its cars being invovled in a horrific accident at a race in Le Mans. Still used sometimes for its modern race cars.
Sinistra (CoS ch. 11): The witch who teaches Astronomy at Hogwarts.
Etym: Feminine form of Latin sinister, originally meaning "left-handed"; has connotations of "strange", "abnormal", the occult, etc. A Slytherin, perhaps?
Sirius Black: Etym: Canis Major, the Dog Star.
Sites of Historical Sorcery: A book which mentions a goblin rebellion at Hogsmeade in 1612.
Skele-Gro: A medicinal concoction used to help regrow or enlarge bones.
Sleeping Draught: A potion that puts the drinker into an uninterruptible sleep.
Slytherin House: One of the four houses of Hogwarts. Its defining virtues are subject to debate: many non-Slytherins believe it selects for evil; the pure-blood faction says it's the house of the "true" wizards; the Sorting Hat and Dumbledore say determination and 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".
Slytherin, Salazar: One of the founders of Hogwarts, and constructor of the Chamber of Secrets. He is rumored to have been a bigot about Muggle-born wizards, but one notes that the Sorting Hat has no problem placing students of Muggle ancestry in Slytherin.
Smeltings: The school that Vernon Dursley attended and which Dudley now goes to.
Etym: Probably invented from smelting.
Smethley, Veronica (CoS ch. 7): A big fan of Lockhart's.
Etym: From the name of an unspecified place, probably derived from Old English smeþe "smooth" + leeah "wood, clearing".
Snape, Severus: The Potions teacher at Hogwarts, and head of Slytherin House, who carries a grudge against Harry merely because Harry's father and his friends almost killed Snape during their school days, and then Harry's father grew up to be a famous martyr, and then Harry helped the convicted murderer Sirius Black escape after Snape had gone to rescue Harry from the aforementioned convicted murderer, not to mention the free pass Harry gets to go around breaking school rules at will.
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.
Sonnets of a Sorcerer: A book which caused everyone who read it to speak in limericks for the rest of their lives.
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. Originally, it was Godric Gryffindor's hat.
Special Award for Services to the School: Awarded to Tom Riddle for unspecified reasons-- possibly for exposing Hagrid as the opener of the Chamber of Secrets.
Spellotape: What Ron fixed his wand with.
Etym: A pun on Sellotape, the British term for Scotch tape.
Spinnet, Alicia (PS ch. 11): A Chaser for the Gryffindor Quidditch team, two years ahead of Harry.
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".
Squib: A person of magical parentage who has no magical powers.
Etym: A term from the 1800s, referring to a firework that produces only a slight explosion. Earlier, applied to persons, could mean mean, insignificant, or paltry.
squid, giant: A denizen of the lake adjoining Hogwarts, something along these lines.
Stan Shunpike [Stanley]:
Etym: From a place name derived from Old English staan "stone" + leeah "wood, clearing". Perhaps used as a reference to the explorer?
Standard Book of Spells, The: Name of a series of basic spell textbooks used at Hogwarts.
Statute of Secrecy: The law stating that wizard activity should be kept secret from Muggle society.
St. Brutus's Secure Center for Incurably Criminal Boys: Where the Dursleys now claim Harry is going to school.
Etym: Can't find anything on any such saint but the name is cognate to "brutal".
Stink Pellets: Things available at Zonko's Joke Shop.
St. Mungo's Hospital for Magical Maladies and Injuries: The current residence of Gilderoy Lockhart, hidden behind the storefront of Purge and Dowse Ltd. in London.
Etym: St. Mungo, proper name Kentigern, is the patron saint of Glasgow. The name is Gaelic, meaning "amiable" or "my dear friend".
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 Ancient Runes: A class Hermione started taking in her third year.
Study of Recent Developments in Wizardry, A: A book in the Hogwarts library.
sugar quills: Wizard candies cunningly disguised as the quills students write with in class.
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.
Swelling Solution: A potion that causes whatever it touches to inflate.
Switch, Emeric (PS ch. 5): Author of A Beginner's Guide to Transformation.
Etym: What it looks like.
Switching Spells: Spells used in Transfiguration.