Password

" No Password, no entry!"

—The Fat Lady in 1995

Passwords are used throughout Hogwarts Castle to restrict entry to certain areas such as house common rooms and teacher offices. Passwords in Hogwarts appear to change on a frequent basis.

During Sir Cadogan's period of guarding the Gryffindor common room, he changed passwords so frequently that Neville Longbottom had to write them down. This list was snatched by Crookshanks and was given to Sirius Black, allowing him entrance.

Passwords were also used as a way to gain to access to the radio programme Potterwatch, as a way to protect it from Death Eaters possibly listening. New passwords were introduced at the end of each broadcast, and could be used to listen to the next one. 

List of previously used passwords

Hogwarts

Gryffindor Tower

1991

• Caput Draconis (Latin for "Dragon's Head")
• Pig Snout

1992

• Wattlebird

1993

• Fortuna Major (Latin for "greater luck")
• Flibbertigibbet
• Oddsbodikins
• Scurvy Cur

1994

• Balderdash
• Banana Fritters
• Fairy Lights

1995

• Mimbulus mimbletonia

1996

• Abstinence
• Baubles
• Dilligrout
• Tapeworm
• Quid Agis (Latin for "How are you")

Ravenclaw Tower

(Instead of a password, a conundrum is posed.)

• Which came first, the phoenix or the flame?
  • Answer: A circle has no beginning.
• Where do Vanished objects go?
  • Answer: Into nonbeing, which is to say, everything.

Slytherin Dungeon

1992

• Pure-blood

Headmaster's Office

Minerva McGonagall using "Sherbet lemon" in 1992.

1992

• Sherbet lemon

1994

• Cockroach cluster

1996

• Acid Pops
• Fizzing Whizbees

1997

• Dumbledore

Unknown

• Lemon drop
• Toffee Eclairs
• Chocolate Frogs

Prefects' bathroom

1994

• Pine-fresh

One-Eyed Witch Passage

• Dissendium

Chamber of Secrets and Salazar Slytherin's Locket

• "Open" (spoken in Parseltongue)
• Speak to me, Slytherin, greatest of the Hogwarts four. (spoken in Parseltongue, to release Salazar Slytherin's basilisk from its nesting place)

Elizabeth Burke's portrait

• Slytherins are Supreme
• Facta, non verba(Latin for Deeds, not words)

Percival Pratt's portrait

• This Password is Absurd

Basil Fronsac's portrait

• Studious Success
• Libraries Liberate

Giffard Abbott's portrait

• Dragon's Egg
• Light against Darkness

Damara Dodderidge's portrait

• Chops and Gravy
• Dashing Cadogan

Boris the Bewildered's portrait

• Forget-Me-Never

Old Posh Baron's portrait

• Fizzy Pop

(Grounds portrait)

• Catweazle

(Dungeons portrait)Edit

• Wanglewort

(Upper Castle portrait)

• Wolf's Bane

PotterwatchEdit

• Albus
• Dearborn
• Fawkes
• Longbottom
• Mad-Eye
• Meadowes
• Moody
• Padfoot
• Phoenix
• Prewett
• Sirius
• Vase
• Wulfric (One of Albus Dumbledore's middle names) 

From Harry Potter Wikia