Tafsir is an arabic word
تفسير which is derived from the root 'fassara' which means to
"explain" or to "expound". It is best understood as 'explanation'
or 'interpretation' of the Qur'an.
Tafsir in the language of the scholars means: explanation and clarification.
It aims at knowledge and understanding concerning the book of Allah, to explain
its meanings, extract its legal rulings and grasp its underlying reasons. Tafsir
explains the 'outer' (zahir) meanings of the Qur'an.
Someone who writes tafsir is a 'mufassir' مفسر which comes from the plural