An original character is a common term often heard on Flipnote Hatena. It is occasionally shortened to "OC."
An original character is a character created that does not come from an existing copyright. In other words, it is a new fictional character. This term was originally used in fanfiction to refer to a brand-new character who was unofficially introduced in a story.
What a Original Character Is
Technically (by definition), an Original Character is any character whom which is not infringing a person's or party's copyright. Simply put, it is a character who is not already in existence or an edit of an already existing character. Art style does not apply.
Original Character Example
Birdie BAI's trademark character was Bai, a pink songbird, not copied or inspired by an existing user. Some fans adapted this character for use in their own Flipnotes, but credited Birdie BAI as the original creator.