Definition

An interjection is a word used to express some strong or sudden emotion of the mind.

For example

Alas! I then have forced away my friend.

An interjection is not the sign of an idea, but merely an expression of emotion. Interjections do not have any definable signification or grammatical construction, but are frequently used in spoken colloquial and informal discourse. Interjections must not be omitted in parsing.

Interjection is derived from the Latin word interjectus meaning thrown between. While interjections can be thrown between the parts of the sentence, they are often placed at the beginning or at the end of a sentence.
The most common interjections express:

• Joy or exultation
  • Hey
  • Hurrah
  • Huzza
  • Surprise
  • Aha
  • Hah
  • Ah
• A wish for attention
  • Ho
  • Halloo
  • Ahem
• Aversion or contempt
  • Fie
  • Pshaw
  • Foh
  • Rats
  • Yuk
• Sorrow, grief, or compassion
  • Alas
  • Woe
  • Alack
  • O
• A wish for silence
  • Hush
  • Mum
• Languor
  • Heigh-ho
  • Heigh-ho-hum
• Laughter
  • Ha
  • Ha ha

Some words used as interjections may also be parsed as verbs, nouns, or adjectives as in the following sentences:

• Strange, cried I.
  
  Strange is normally an adjective. Here it is used as an interjection and is equivalent to It is strange.
• Behold! How well he bears misfortune’s frowns!
  
  Behold is a verb. Here it is used as an interjection and is equivalent to behold ye.