Written language or written form (as opposed to spoken language); the style and expressions used in writing.
書き言葉では短縮形を使わないことが多い。
In written language, contractions are often not used.
彼のエッセイは書き言葉としてとても読みやすい。
His essay is very easy to read as written language.
話し言葉と書き言葉は語彙や文法が異なることがあるので注意が必要だ。
Be mindful that spoken and written language can differ in vocabulary and grammar.