Tan Suffix
The Japanese suffix Tan (たん?) is a mispronunciation of Chan (ちゃん?), an informal, intimate, and diminutive honorific suffix for a person used for friends, family, and pets. In this case, the mispronunciation is used intentionally to achieve the contrived cute or charming effect that is commonly associated with its use by young children and is also sometimes added to the names of non-mascot characters. The personifications as a whole are commonly simply called mascots or mascot characters and as such the Tan suffix itself means nothing outside its role as an honorific and its implications of cuteness. Normal suffixes including San, Chan, and Kun are also used with OS-tan depending on the character and the speaker's preference or omitted entirely.
Read more about this topic: OS-tan