Haadith in Arabic is used as opposite to Qadeem. Qadeem means something without a beginning which obviously is none but Allah Ta'ala. The opposite of Qadeem is Haadith which means something with beginning. In other words it means ‘Created (Makhlooq). As per this meaning, the Qur'an is the word of Allah hence is Qadeem, whereas Hadith means that word which is created and since Rasulullah (Sallallahu Alaihi Wasallam) himself is among Makhlooq, so his statements are called Hadith.