Board Thread:Theories/@comment-30473998-20170816133851/@comment-25322628-20170817002433

An Abnormal Titan is best defined by its behavior when it comes to attacking, hunting or being around humans. They can perform actions that humans find unusual for a Titan, such as running past a small number of humans toward a larger gathering of humans with such speed that they look like they're crazy.

Another example could be speaking (unless they're not a skilled Titan Shifter), recalling some form of memory.

Or just having some sort of ability that isn't present in other Titans.