In terms of the Shari’ah the two words, Imaan and Islam , are inextricably interwoven. The Shar’i meaning of the one is incomplete without the meaning of the other. The two terms are complementary. The one is a necessary corollary of the other. The existence of the one is dependent on the existence of the other. Negation of the one implies the automatic negation of the other. A proper understanding of Imaan is, therefore, possible only by a study of the Shar’i definition of both words and their interrelationship to one another.
Literal Meanings

Imaan literally means:

To verify, to accept, to attest with the heart.

The acceptance by the heart with conviction is termed Imaan in the literal sense. Hence, Imaan is a state or an internal (Baatin) condition.

Islaam literally means:

“To submit oneself to another; to make oneself lowly in the presence of another .”

In the literal sense. Therefore,Islam is an external (zaahiri) state in which a display or demonstration of the internal (baatini) condition is made.

These literal meanings of the two words have their share in the Shar’i definition of Imaan and Islaam. Basing the technical (i.e. the Shar’i ) meanings on the literal meanings. Imaam Abu Muhammad Mas’ud Baghawi Rahmatullah Alay says:

“Nabi (Sallallahu Alayhi wasallam) defined Islaam as the name for external acts and Imaan as the name of the internal beliefs.”

kitaabul Imaan_ERead