Professor Horace E. F. Slughorn

"Harry, Harry, just the man I was hoping to see! I was hoping to catch you before dinner! What do you say to a spot of supper tonight in my rooms instead? We're having a little party, just a few rising stars, I've got McLaggen coming and Zabini, the charming Melinda Bobbin — I don't know whether you know her? Her family owns a large chain of apothecaries...."
     --Horace Slughorn (HBP11)

Hogwarts Potions Master until 1981, then again from 1996; Head of Slytherin house 1997-1998.

Age: Between 80 and 150 [older than 80 (HBP9) and younger than Dumbledore (HBP4)].
Birthyear: undetermined.
Possible name meaning: slughorne=A hereditary feature or characteristic of a family or race (The Word Museum, 2000). Note: Slughorn signed his invitations as 'H.E.F. Slughorn' (HBP7).

Appearance: short stature, shiny, bald head, prominent eyes, large belly, enormous silvery walrus-like moustache (HBP4&7). Wears lavish, old-fashioned clothes: waistcoats with gold buttons during the day and luxurious velvet smoking jackets when at home. When he was younger he had thick, shiny, straw-colored hair (HBP17) and a gingery-blond moustache (HBP23). During the Battle of Hogwarts he even dueled Voldemort whilst wearing an emerald-green pair of silk pajamas (DH36).

Hogwarts: Began teaching about the same time as Dumbledore; retired at the end of the 1980-81 school year. Dumbledore and Harry talk him into returning to teach Potions again in the summer of 1996. Created "Slug Club" of favored students.
House: Former and present Head of Slytherin House.
Field: Potions.
Other skills: Presumably also talented at transfiguration because he was able to turn himself into an overstuffed armchair at a moment's notice. Slughorn is even better at name dropping than Gilderoy Lockhart. He has an "uncanny" knack for picking and mentoring students who will make their mark on the Wizarding world; he fosters these connections to the famous, successful and the powerful.
Office: Below the hospital wing (HBP18).
Location of home: Unknown. He is found by Dumbledore and Harry "borrowing" a Muggle house in the Village of Budleigh Babberton.
Interests: Enjoys his "comforts:" rich food, crystallized pineapple, chocolates, and fine wine.
Felix Felicis: Slughorn has partaken of the Felix Felicis potion twice (2 Tbsp. with breakfast) in his life: when he was 24, and when he was 57. He clearly has very fond memories of those two days (HBP9).

Profile
At the beginning of Harry's 6th year at Hogwarts, Dumbledore and Harry convince a genial wizard named Horace Slughorn to come out of retirement to teach Potions.

Ever since Voldemort regained his powers, Slughorn had lived in fear that he will be confronted by Death Eaters and pressured to join them. For over a year he constantly moved from place to place, inhabiting the homes of muggles who leave their houses unattended.

Slughorn is charming and jovial, and genuinely admired Harry's mother, Lily Evans. However, Harry is wary of such fawning attention, and throughout the school year maintained a distance that must have frustrated the Potions Master. Who can blame Harry, after meeting so many people who would use him and his fame for their own ends?

An "old friend and colleague" of Dumbledore's, Slughorn is one of the first Slytherins that Harry has met who has no prejudices against muggle-born or half-blood wizards. Indeed, Lily Evans was one of his favorite students.

Slughorn's secret
But Slughorn has a shameful secret -- it was he who told Tom Riddle about some of the darkest magic of all: Horcruxes. Tom had already opened the Chamber of Secrets and been responsible for at least 4 deaths (HBP). Perhaps this is why Slughorn is so very shaken when Dumbledore is killed (HBP29).

Slughorn fought in the Battle of Hogwarts whilst wearing emerald-green silk pajamas, where he dueled with Voldemort (DH30, 36).