The type of music that started as a subgenre of trance and is still considered so by many people. Trance fans usually call is happy hardcore. However nightcore fans prefer to not be associated with trance and don't like the idea of naming a "nightcore" as anything else.

Nightcore is characterized by a sped-up melody (sometimes), fast rhythmic beat (usually), and ALWAYS higher than normal pitch. Almost all nightcore music are original songs nightcored (remixed into nightcore) by nightcore fans.

Very popular is a belief that nightcore is a nickname of a person who remixes the songs. In reality anyone who knows how to use a computer can nightcore a song and upload it on Youtube.

Genre that originated from trance and/or techno, and eventually got a life on its own.

Pure Nightcore: VERY fast beats (sometimes so extremely fast, that you cannot count them), and very high pitch (almost chipmunks). Song usually lasts about 2 to 3 minutes. Often described as "cute".
Example: Kiss me by Schule321

Bass Nightcore: fast beats, the main melody (and voice) are pitched up, but there's very low bass added; so the contrast between very high and very low is created.
Example: Kiss me by GrizzNightcore

Trance Nightcore: fast beats, high pitch, but less signing and more melody; filled with repetition; very often there is only one verse or a sentence that is repeated throughout the song. Usually it lasts at least 5 minutes, often more.
Example: Kiss me by Nightcoremania

Almost always refers to a song that was initially trance or dubstep, sped up to the point where the lyrics sound like they are being recited by a hurried chipmunk, featuring a strong beat which is is audible but barely discernible. The song is then posted on YouTube, usually giving no credit to its initial creator, accompanied by a visual of an anime girl (or multiple anime girls).

As yet unrecognized genre of the music that originated on the Internet, more specifically YouTube. Some people claim to be the creator of it, but since the original account is no longer active (and no one even remembers what was the nickname of the person) those claims cannot be proved.

The main characteristic of nightcore is its high pitch often accompanied by fast beat. Very often songs that were remixed into trance, are later on remixed one again into nightcore and so many nightcore songs have trance-like characteristics.

Even though almost any song of any genre can be nightcored, trance and techno songs, as well as electro music and all types of dance music (here especially eurodance) are most often used. Some rock and pop songs are also nightcored. Rap and metal are the two genres that are heavily avoided, and there are very few songs remixed into nightcore.

Nightcore is an awesome music genre which is some sort of trance but sped up and high pitched.

The name of a electronica duo from Norway. Nightcore consists of Thomas S. Nilsen and Steffen Ojala Søderholm from Norway, Alta.

On November 6, 2006 a youtube user nicknamed Cuteangel43802 uploaded the AMV called:
Kingdom Hearts: Dont You: Nightcore

And this became the beginning of the nightcore style. The word “nightcore“ was included in the title, as well as under the video as a “music artist“, rather than “singer“ or “group“, and so it was interpreted to be nickname of the person remixing (i.e. high pitching) the song.

Eventually anyone high-pitching the songs would call themselves nightcore (but to distinguish themselves they would add a word, letter or a number before or after the word), and as the amount of people doing it increased, the meaning transferred to the song genre, rather than the person.

Nightcore can include any genre, because the word itself means “high-pitch“, so it is much broader term than “happy hardcore“ and although some “happy hardcore“ songs can be also called “nightcore“, not all “nightcore“ songs can be called “happy hardcore“.

Wikipedia defines “nightcore“ as a name of a band, but whether there is a connection between the youtube nightcore community and the band is unknown. Possibly the name was directly or indirectly taken from the name of the band, but there is no definite proof for this. It may have been as well a coincidence.