GOTHIC ARCHITECTURE
Today, the word Gothic primarily describes a style of European architecture which flourished from the twelfth through the sixteenth centuries, though the word seems originally to have referred to any non-classical (Greek or Roman) architecture.
Gothic architecture used pointed arches and vaults, flying buttresses, narrow spires, stained glass windows, intricate traceries, and varied details; its upward movement was meant to suggest heavenward aspiration.
The words Goth and Gothic also described the Germanic tribes (e.g., Goths, Visigoths, Ostrogoths) which sacked Rome and also ravaged the rest of Europe in the third, fourth, and fifth centuries. From this source, the words came also to mean barbarian, barbarous, and barbaric. By the eighteenth century in England, Gothic had become synonymous with the Middle Ages, a period which was in disfavor because it was perceived as chaotic, unenlightened, and superstitious. Renaissance critics erroneously believed that Gothic architecture was created by the Germanic tribes and regarded it as ugly and barbaric. This erroneous attribution continued through the eighteenth century.
As a result of an upshot of interest in the Middle Ages, Gothic architecture experienced a revival in the late eighteenth century; Horace Walpole rebuilt Strawberry Hill as a medieval castle and William Beckford spent a fortune on his medieval, elaborate imitation, Fonthill Abbey. The revival flourished in the nineteenth century and Gothic buildings were constructed throughoug England.
GOTHIC LITERATURE
The English Gothic novel began with Horace Walpole's The Castle
of Otranto (1765), which was enormously popular and quickly
imitated by other novelists and soon became a 
recognizable genre. To most modern readers, however, The Castle
of Otranto is dull reading; except for the villain Manfred,
the characters are insipid; the action moves at a fast clip with
no emphasis or suspense, despite the supernatural manifestations
and a young maiden's flight through dark vaults. But contemporary
readers found the novel electrifying original and thrillingly suspenseful,
with its remote setting, its use of the supernatural, and its medieval
trappings, all of which have been so frequently imitated and so
poorly imitated that they have become stereotypes. The genre takes
its name from Otranto's medieval–or Gothic–setting; early Gothic
novelists tended to set their novels in remote times like the Middle
Ages and in remote places like Italy (Matthew Lewis's The Monk,
1796) or the Middle East (William Beckford's Vathek, 1786).
What makes a work Gothic is a combination of at least some of these elements:
- a castle, ruined or intact, haunted or not,
- ruined buildings which are sinister or which arouse a pleasing melancholy,
- dungeons, underground passages, crypts, and catacombs which, in modern houses, become spooky basements or attics,
- labyrinths, dark corridors, and winding stairs,
- shadows, a beam of moonlight in the blackness, a flickering candle, or the only source of light failing (a candle blown out or an electric failure),
- extreme landscapes, like rugged mountains, thick forests, or icy wastes, and extreme weather,
- omens and ancestral curses,
- magic, supernatural manifestations, or the suggestion of the supernatural,
- a passion-driven, wilful villain-hero or villain,
- a curious heroine with a tendency to faint and a need to be rescued–frequently,
- a hero whose true identity is revealed by the end of the novel,
- horrifying (or terrifying) events or the threat of such happenings.
The Gothic creates feelings of gloom, mystery, and suspense and tends to the dramatic and the sensational, like incest, diabolism, and nameless terrors. Most of us immediately recognize the Gothic (even if we don't know the name) when we encounter it in novels, poetry, plays, movies, and TV series. For some of us--and I include myself, the prospect of safely experiencing dread or horror is thrilling and enjoyable.
Elements of the Gothic have made their way into mainstream writing. They are found in Sir Walter Scott's novels, Charlotte Bronte's Jane Eyre, and Emily Bronte's Wuthering Heights and in Romantic poetry like Samuel Coleridge's "Christabel," Lord Byron's "The Giaour," and John Keats's "The Eve of St. Agnes." A tendency to the macabre and bizarre which appears in writers like William Faulkner, Truman Capote, and Flannery O'Connor has been called Southern Gothic.
THE OXFORD ENGLISH DICTIONARY DEFINTION
The Oxford English Dictionary (OED) does not list the literary meaning of Gothic, though it does list the other meanings I have discussed. The OED differs from the dictionaries we use most of the time; it traces words historically, that is, it lists the first appearance of a word in English and traces its usage and changes over time. I have included the relevant definitions of Gothic for those of you who are interested in words and language or who might just be curious.
Dates for when a word first appeared in writing are from the OED unless I state otherwise.