Pansy Parkinson: Etym: Viola tricolor aka heartsease. Signifies "thoughts", and the common name is derived from the French for "thought".
Paracelsus (PS ch. 6): The pseudonym of Philippus Aureolus Theophrastus Bombastus von Hohenheim (1493-1541), physician and alchemist, who established the role of chemistry in medicine, and was one of the first advocates of an empirical approach that became modern scientific method.
Parkinson, Pansy (PS ch. 7): A Slytherin in the same year as Harry.
Etym: From a diminutive of Peter.
Parvati Patil: Etym: EB: "The benevolent aspect of Shakti." Depending on which thread of Hinduism you follow, Shakti is either (a) the wife of Shiva, (b) a wife of Shiva, (c) the feminine, creative aspect of Shiva, (d) a personification of positive energy in general, or (e) all of the above but it doesn't matter since all definable things are illusion anyway.
Patil, Padma: A student of unknown house, in the same year as Harry.
Etym: See below.
Patil, Parvati: A Gryffindor, same year as Harry.
Etym: From the Pictish peit "portion (of land)" and Gaelic tulach "hill". Alternatively, also a surname of Indian origin.
Peeves the Poltergeist: An obnoxious ghost inhabiting Hogwarts, teasing the students and irritating the staff and other ghosts. Controllable only by the teachers and the Bloody Baron.
Etym: Invented, as far as I can tell, from peeve.
Percy Weasley: Etym: A family name that dates back to William de Perci, a comrade of William the Conqueror.
Perenelle Flamel: Etym: I seem to recall this is a name for some kind of flower, but can't find any info. Anyone?
Perks, Sally-Anne (PS ch. 7): A student of unknown house, in the same year as Harry.
Etym: Variation of park, which in the Middle Ages meant a large enclosed area used for game hunting.
Petrificatus Totalus: The incantation for the Body-Bind.
Etym: Latin-ish back-formation from petrify and total.
Petunia Dursley: Etym: From petun, "tobacco"; the flower is in the tobacco family.
Philosopher's Stone: A substance representing the ideal combination of the essences of all elements, sought by alchemists down the ages, thought to make it possible to do all sorts of magical things. In these books, an actual stone which can be used to make the Elixir of Life.
phoenix: A mythical bird which lives 500 years and then dies, only to be reborn in a magical fire. Phoenix feathers are used as wand cores.
Phyllida Spore: Etym: Probably from Greek phyllos, meaning "leaf".
Piers Polkiss: Etym: A French variation of Peter.
Pince: The librarian at Hogwarts.
Etym: Variation of Pinch, a nickname for a chirpy person. Really.
Platform 9 3/4: The platform for the Hogwarts Express at King's Cross Station.
Polkiss, Piers (PS ch. 2): Dudley Dursley's best friend and chief accomplice in tormenting Harry.
Etym: No etymology.
Pomfrey, Poppy: The nurse at Hogwarts, for whom treating broken arms, catatonia, poison, and the like is a typical day's work.
Etym: Welsh name from the given name Humphrey, from the Germanic huun "bear cub" + frid "peace". (Alternately, OEW translates it as "peace through force".) St. Humphrey was a 9th century bishop of Therouanne.
Poppy Pomfrey: Etym: Various flowers of the genus Papaver. Signifies consolation or oblivion. Also, in the UK, worn to commemorate veterans of the World Wars.
Potions: A standard class at Hogwarts; taught by Professor Snape.
Potter, Harry: An orphaned wizard, unique among even his magically talented classmates in that dark forces really are constantly attacking him and causing him to miss class. Wand: 11", supple, holly and phoenix feather.
Etym: Stated by Rowling to be a name she just liked.
Potter, James: Harry's father, killed by Voldemort. In school, he was a Gryffindor and became Head Boy. Wand: 11", pliable, mahogany.
Potter, Lily: Harry's mother, killed protecting Harry from Voldemort. She came from a Muggle family, was a Gryffindor and became Head Girl when she was at Hogwarts. Wand: 10 1/4", swishy, willow.
prefect: A fifth-year or later student appointed by the teachers to take some responsibility for other students. There appears to be at least one prefect for each of the Hogwarts houses.
Prewetts (PS ch. 5): Wizards killed by Voldemort.
Etym: Diminutive form of a name from Middle English prou(s) "brave", "valiant".
Privet Drive: The street in Little Whinging where the Dursleys live. Privet is commonly used in hedges-- very much a feature of dull, conformist suburbia.
Pucey, Adrian (PS ch. 11): A Chaser for the Slytherin Quidditch team.
Etym: From a town in Berkshire.
Pumpkin Pasties: A wizard candy, probably some kind of pastry with pumpkin filling.
Put-Outer: A magical device that can supress lights.