There are numerous plot devices which authors of fiction employ which are known by specific terms, some more light-hearted than others. Here is a discussion of a small selection of these tropes: the Sampo, the Big Dumb Object, the MacGuffin, Alien Space Bats, Chekhov's Gun, the Red Herring, and Deus Ex Machina.
The first of these is the Sampo. The Sampo is a term derived from Finnish mythology, specifically the tale 'Sampo the Magic Mill'. To digress, the tale is about two brothers. Vainamoinen is a musician, and Ilmarinen is a blacksmith. They attempt to court the same woman, Aino- the daughter of the powerful and apparently fiendishly evil Queen Louhi of Pohjala. The two brothers are set magical tasks to win Aino's hand in marriage while the Queen attempts to thwart their plans (because she is evil, obviously).
Ilmarinen eventually wins Aino's hand in marriage by discovering the magical three words of a giant, a magical formula which allows him to create a Magic Mill. This Mill can make flour, salt and gold out of thin air, and is called the Sampo. Ilmarinen is allowed to marry Aino as the Sampo brings good fortune and prosperity to Queen Louhi and Pohjala (personally I would have ditched Aino and taken the Mill away in the first place, didn't they realise their mother in law was evil?). Sadly, shortly after marrying Ilmarinen, Aino sickens and dies (bad luck). The brothers decide to steal the Mill back from Queen Louhi and bring it to their own lands across the sea. Angered by the theft, Queen Louhi prays to the God Ukko, who listens to her and sends a storm which destroys the brothers' boat and sinks the Sampo to the bottom of the ocean. The brothers are saved (an outcome which they barely deserve after their sentimental stupidity) and the Sampo is stuck at the bottom of the ocean where it is jammed on producing the second product, salt. Hence the reason why to this day the seas and oceans are full of salt.
Essentially, the Sampo is an important artefact or object, usually magical but sometimes not, which is at the centre of a narrative and is the driving force for the actions of the principle characters. Another famous example of this is the 'One Ring' carried by Frodo Baggins in J.R.R Tolkien's Lord of the Rings.
Similarly, the more colloquial phrase Big Dumb Object is a term used primarily in regard to science fiction. It is used to describe an artefact or physical object similar to the Sampo which possesses extreme or unusual properties and/or powers, often physically imposing or grandiose in scope. An example of this from fiction/cinema would be the black monolith in 2001: A Space Odyssey which apparently triggers the process of evolution.
The MacGuffin (McGuffin, maguffin) can be an object, or a concept or goal which the protagonists of a plot pursue or ruminate upon. It differs from the Sampo and the BDO in terms of not always being a physical object, and often being a plot device which is mysterious, not fully explained or eventually superfluous to the story. It is used to drive plot and character action, and is not always the focal point of the narrative. The term originated in cinema.
Examples of it include the meaning of the word 'Rosebud' in Orson Welles' Citizen Kane and the Rambaldi device in the TV series Alias. In literature, one example of it is the 'Samizdat' entertainment cartridge in David Foster Wallace's novel Infinite Jest.
Alien space bats is another colloquial phrase specific to alternate history fiction. It refers to an event or detail in the plot which creates a point of divergence from real world history. Usually, the term is employed to describe something over-dramatic or implausible in that context. The term was coined by Alison Brooks, who was commenting upon the Nazi Operation Sea Lion plan- the plot to invade the British Isles during the Second World War. Brooks humorously remarked that “the only way it could be successful was if alien space bats helped the Nazis.”
Chekhov's Gun is a term derived from the Russian writer Anton Chekhov, linked to presentiment and foreshadowing. This is a narrative technique whereby authors introduce a seemingly irrelevant or superfluous detail, description or event early in the story. Initially that element appears unimportant, before assuming greater significance and importance later in the narrative. The example Chekhov used would be to describe the presence and location of a gun within a room early on in a story, which later on is used and fired by one of the protagonists.
It is a neat and sophisticated way to construct a narrative structure and introduce plot twists into the story, which then make plausible sense and fit together with the narrative beforehand. A good example of this technique would be the film Shutter Island directed by Martin Scorsese (based on the novel by Dennis Lehane).
The Red Herring is almost the opposite of Chekhov's Gun (C's G is essentially an apparent insignificant detail which later becomes significant), instead the Red Herring is a plot device which misleads and distracts the reader from the real truth of the plot. The emphasis than an author places on the Red Herring convinces (if done so successfully) that a particular character, event or element is significant when it is, in fact, not. It is often used in crime and mystery fiction to create intrigue and suspense. The term was invented in its present usage by the radical writer William Cobbett.
Deus Ex Machina translates from Latin as 'God from the Machine'. This is a narrative device used where a problem or situation which seemed unsolvable or unrecoverable is resolved by the contrived intervention of a new event, character, ability or object. It was used often in ancient Greek drama where a crisis was solved by the intervention of an all-powerful God. It is often criticised for being a clumsy method of solving plot holes or perilous situations, and carries the risk of challenging the reader's suspension of disbelief.
Here are plenty of examples of it occurring in fiction!