Any system of writing that uses graphemes (as opposed to phonemes) which denote meaningful elements of speech; in other words, the symbols are unrelated to the spoken sounds.
A grapheme is the smallest unit used in describing the writing system of any given language, originally coined by analogy with the phoneme of spoken languages. A grapheme may or may not carry meaning by itself, and may or may not correspond to a single phoneme.
Looking at a sentence like this one, I understood why the heptapods had evolved a semasiographic writing system like Heptapod B; it was better suited for a species with a simultaneous mode of consciousness. For them, speech was a bottleneck because it required that one word follow another sequentially. With writing, on the other hand, every mark on a page was visible simultaneously. Why constrain writing with a glottographic straitjacket, demanding that it be just as sequential as speech? It would never occur to them. Semasiographic writing naturally took advantage of the page’s two-dimensionality; instead of doling out morphemes one at a time, it offered an entire page full of them all at once.
Definition from Wiktionary, Creative Commons Attribution/Share-Alike License (via Wordnik)