Researchers working for the ACCESS Project at Colorado State University (http://accessproject.colostate.edu/) defines it this way:
"A learning disability in which a person’s ability to communicate in writing is substantially below the level normally expected based on the individual’s chronological age, measured intelligence, and age appropriate education."
Researches have recognized signs of written expression as:
Inability or low level of ability to organize words into meaningful thoughts, resulting in sentences that lack cohesion.
Inability to write spontaneously.
Poor grammar, spelling and punctuation errors.
Poor handwriting including poorly formed letters and numbers.
Inability to remember letter sequences in common words