Mary Sue
A Mary Sue is a pejorative name for a certain type of character.
Overview
Coined by Paula Smith in 1973 in her satirical Star Trek fanfic "A Trekkie's Tale", the term "Mary Sue" has come to be used for an over-the-top badly written character that has broken the suspension of disbelief of a story to its detriment. The archetypical Mary Sue character is shallow and static, highly idealized, does not react to their situation in a way that makes narrative or psychological sense, and has traits that are extraordinary or out of place in the context of the story's setting. What exactly constitutes as "over-the-top" or "out of place" will of course vary from one setting to the next: a New Century Sue, for instance, might be a beautiful, intelligent, charismatic prodigy Zoid Warrior with a custom Ultimate X König Wolf, an Organoid and a gratuitous tragic backstory, who can beat Bit and Vega at the same time with her eyes closed and takes agency away from the canon characters, reducing them to jobbers, worshippers or love interests.
Minutiae
Most would agree on the broad definition outlined in the above section, but much controversy exists over the subjective finer details. The term "Mary Sue" is generally used among the fanfiction and RPG crowd, and some purists will insist the term only applies to fanfic and RP: others will argue the term applies to original fiction as well, pointing to characters such as Twilight's Bella Swan, Star Trek's Wesley Crusher or Ayla of Earth's Children as examples of Canon Sues. Mary Sues are often (but not always) surrogates for the author, and may share their personality, interests, political views, religious affiliation, sexual orientation, etc. A male Sue can be referred to as a Gary Stu, and although they certainly exist there is a regrettable tendency for them to not be criticized as harshly as their female counterparts. Anti-Sues are the extreme opposite of a typical Mary Sue - too flawed to be interesting, instead of being too perfect to be interesting.
Analysis
It's important to note that there is nothing inherently wrong with escapism or catharsis in fiction. Mary Sue is not a list of character traits: it is the disease, not the symptoms, and a character can have some Sue-like traits without necessarily being a Sue. For example, Bit is coddled by the plot and has a special Zoid, but these traits are balanced out by his buffoonish antics and legitimate character flaws. "Mary Sue" can be a loaded word: beware of those who use it as a shorthand for "any character I don't like who I think is too powerful or important."
