That's kind of like trying to define 'language' when we're speaking of paleo-words uttered maybe (or maybe not) by our earliest ancestors (pre-writing but post-Wernicke's area of the brain). Is language just the spoken word, or does it include body language, facial gestures, inflections?
So to text--is it just the black-and-white leximes and phonemes of language, that must be transcribed to be understood. Or does it include 'communication'--pictures, emoticons, acronyms more easily understood by all?