"Jabberwocky" is a nonsense poem contained within the text of Lewis Carroll's "Through the Looking Glass". The poem describes the hero's father warning him to avoid various dangerous creatures, but the hero takes his sword, seeks out the Jabberwock, kills it, and returns home with its head, to his father's great joy. The poem is characterized by the use of numerous nonsensical or "made-up" words which have no inherent meaning, but convey images of danger, onomatopeia, and various other descriptive qualities.
It is not entirely clear what the Jabberwock is; the poem describes it as having "jaws that bite" and "claws that catch" as well as "eyes of flame". It is also described as "whiffling" in its motion and "burbling" as it does so, but these are nonsense words, which based on the tone of the poem are clearly intended to be great and terrible traits for a creature to possess.
John Tenniel provided an illustration of the Jabberwock; it is attached below. Note, however, that based on the description above, the appearance of the creature is highly open to interpretation; it may be a normal animal, or it may even be an allegorical "monster" such as jealousy or doubt.