The word Deen is another Arabic word which is roughly translated as
religion. To know what Deen is we actually should know what the word
religion means in English.

"Religion: a set of beliefs concerning the cause, nature, and purpose of
the universe, esp. when considered as the creation of a superhuman
agency or agencies, usually involving devotional and ritual observances,
and often containing a moral code governing the conduct of human
affairs." - dictionary.com

The most important point to consider here is that the word religion
usually is connected with the man's personal life, especially in the
area of worship. This gives us the notion that the God, in which a
person believes, guides him only in his personal life in the field of
worship.

If God is the Supreme the Almighty, the One Who has created the whole
universe and the Sustainer of it, then He must be guiding us in every
aspect of our life. Be it in the areas of business, politics, moral
conduct or how to protect our family, our society, our country etc. In
short the complete way of life.

So the word Deen is nothing but the "complete way of life". There is a verse in Quran which says: The Deen acceptable by the
Almighty is Islam. Which actually means: The (complete) way of life
acceptable by the Almighty is to submit your will to the Will of
Almighty God.