Chibi is a Japanese slang word literally translating to "short person" or "small child".
In English, the term chibi mostly refers to the "super deformed" style of drawing characters: oversized heads and eyes, small bodies with stubby limbs, and typically minimal fine detail.
It is sometimes used to describe aged down child versions of characters as well, but this definition should not be used for tagging chibi (unless the above also applies).