Expecto Patronum.
The spell Expecto Patronum is a spell Harry learns to cast in his 3rd year at Hogwarts in JK Rowlings famous series. The spell conjures a patronus that drives away dementors by countering their need to feed on unhappiness with a happy memory. Harry Potter's patronus in particular takes the form of a white stag.
The correct name for this spell is the "Patronus Charm" It is a very advanced and difficult spell. To cast it, you must concentrate very hard on your happiest memory while saying the incantation "EXPECTO PATRONUM". This conjures a silvery-white animal (e.g. Harry's is a Stag) Which will drive away dementors--they will feed off it, instead of you.
Harry Potter's patronus takes the shape of a stag, the animagus and patronus form of his father. The Patronus spell is used against Dementors but can also be used as a method of communication.
Severus Snape
A patronus Spell: Expecto Patronum!
The correct spelling is patronus when referring to the magical charm
Because the stag is Harry's patronus.
The spell Expecto Patronum is used for calling your Patronus in Harry Potter.
Dumbledore's patronus was a phoenix. He used his patronus in Goblet of Fire, chapter 28 to send a message.
The animal is particular to each person. For example, while Harry's patronus is a stag, Luna's is a rabbit and Hermione's a badger. It's an animal that is important to you, or most resembles your personality.
It is not a hologram, but a patronus (spell to defend against Dementors and send messages to other wizards in no time). The deer patronus belongs to Snape, and very specifically to Lilly Potter (Harry's mum). Snape loved her so much that he took over her patronus as a sign when she was killed by Voldemort. The patronus represents itself to Harry and helps him find the real sword of Gryffindor.
A patronus in snake form.