- Onkelos
- Ordinance
- Ordinance of 1787
- Oregon
- Oregon (Illinois)
- Oregon, University of
- Oregon Brook-Trout
- Oregon City
- Oregon Question
- Oregon River
- Oregon Robin
- Oregon Snowbird
- Oregon Sturgeon
- Oregon Trail, The
- O'Reilly, Count Alexander
- Orinoco
- Oriole
- Orion
- Orthopoda
- Orthoptera
- Ortigueira
- Ortler
- Ortolan
- Orton, Edward
- Orton, James
- Ostend Manifesto
- Ottendorfer, Oswald
- Oven-bird
- Overbeck, Johannes Adolf
- Overbeck, Johann Friedrich
- Owen, Robert
- Owen, Robert Dale
- Owensboro
- Owens College
- Owens Lake
- Owen Sound
- Owens River
- Owl
- Owlet-moth
- Oxenstierna, Axel
- Oxytocics
- Oxyuris
- Oyer
- Oyer and Terminer
- Oyez
- Oyo
- Oyster
- Oyster Bay
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- Palfrey, Francis Winthrop
- Paraná
- Paraná (State)
- Parker, Alton Brooks
- Parker, Edward Harper
- Parker, Foxhall Alexander
- Parker, Francis Wayland
- Parker, (Horatio) Gilbert
- Parker, Horatio William
- Parker, Hyde
- Parker, Joel (jurist)
- Parker, Joel (politician)
- Parker, Joseph
- Parker, Louis Napoleon
- Parker, Matthew
- Parker, Peter (officer)
- Parker, Peter (missionary)
- Parker, Theodore
- Parker, Thomas Jeffery
- Parker, Willard
- Parker, William
- Parker, William Kitchen
- Parliament, Houses of
- Parliamentary Law
- Parliament of Dunces, The
- Parliament of Fowls, The
- Parloa, Maria
- Parris, Samuel
- Passion-flower
- Passionists
- Passion Play
- Passover
- Passow, Franz
- Passport
- Pastor
- Pastor, Ludwig
- Pastoral Epistles
- Pastoral Poetry
- Pastoral Staff
- Pastoral Symphony, The
- Pastorels
- Pastor Fido
- Pastorius, Francis Daniel
- Pasturage
- Pasture
- Patron
- Patronage, Ecclesiastical
- Patton, Francis Landey
- Patuxent
- Pátzcuaro
- Patzum
- Pau
- Pauer, Ernst
- Paul (apostle)
- Paul (popes)
- Paul I., Petrovitch
- Paul, Apocalypse of
- Paul, Charles Kegan
- Paul, Hermann
- Paul, Lewis
- Paul, Oskar
- Paul, Vincent de
- Paul, Vision of
- Paul and Virginia
- Paul Clifford
- Paulding, Hiram
- Paulding, James Kirke
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- Peabody, Cecil Hobart
- Peabody, Elizabeth Palmer
- Peabody, Ephraim
- Pearl (poem)
- Pearl Ash
- Pearl Harbor
- Pearlite
- Pearl Millet
- Pearl Oyster
- Pearl River
- Peasant War
- Peck, Harry Thurston
- Pegu (division)
- Pegu (capital)
- Penjamo
- Penjdeh
- Penn, John (1729-95)
- Penn, John (1741-88)
- Penn, Richard
- Penn, Thomas
- Penn, Sir William
- Penn, William
- Pennacook
- Pennamite-Yankee War
- Pennant
- Pennant, Thomas
- Pennant's Marten
- Pennant-winged Nightjar
- Penn College
- Pennell, Joseph
- Penni, Gianfrancesco
- Pennine Alps
- Pennine Chain
- Pennsylvania
- Pennsylvania, Historical Society of
- Pennsylvania, University of
- Pennsylvania, Western University of
- Pennsylvania College for Women
- Pennsylvania Dutch
- Pennsylvania-German Society, The
- Pennsylvania State College
- Penny
- Pentateuch
- People's Party
- Peoria (tribe)
- Peoria (city)
- Perfectionists
- Perimeter
- Perinæum
- Period
- Periodical
- Periodic Function
- Perkins, Frederic Beecher
- Perkins, George Henry
- Petri, Olaus
- Petrie, William Matthew Flinders
- Petrifaction
- Petrified Forests
- Petrobrusians
- Petrified Wood
- Petrographic Province
- Petrography
- Petrolatum
- Petrolea
- Petroleum
- Petunia
- Peucer, Kaspar
- Peuerbach, Georg von
- Peutinger, Konrad
- Peutingerian Table
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- Pfeiffer, Franz
- Pfeiffer, Ida
- Pfeil, Joachim Friedrich
- Pfeufer, Karl von
- Pfister, Albrecht
- Pfizer, Gustav
- Pfleiderer, Otto
- Pflüger, Eduard Friedrich Wilhelm
- Pflugk-Harttung, Julius von
- Pforzheim
- Pfuhl, Johannes
- Pharisees
- Pherecrates
- Phila
- Philadelphia (Asia Minor)
- Philadelphia
- Philadelphia Academy of Natural Sciences
- Philadelphus
- Philæ
- Philander
- Philemon (poet)
- Philippics
- Philippine Islands
- Philippine Languages
- Philippons
- Phœnician Language
- Phœnissæ
- Phœnix (bird)
- Phœnix (Alabama)
- Phœnix (Arizona)
- Phrynichus
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