The following is an excerpt under the heading of "Islam" from the Dictionary of Islam ©1886 by Thomas Patrick Hughes. In order to retain the old-style flavour of this definition, we have neither edited nor tampered with it.

Resignation to the will of God. The word generally used by Muhammadans themselves for their religion. 'Abdu 'l-Haqq says it implies submission to the divine will; and Muhammad explained it to mean the observance of the five duties: (1) Bearing witness that there is but one God; (2) Reciting the daily prayers; (3) Giving the legal alms; (4) Observing the Ramadan or month's fast; (6) Making the pilgrimage to Makkah once in a lifetime. 
 

In the Qur'an the word is used for doing homage to God. Islam is said to be the religion of all the prophets from the time of Abraham, as will appear from the following verses (Surah iii. 78,79) : - 

"SAY: We believe in God and in what hath been. sent down to Abraham, and Ishmael, and Isaac, and Jacob, and the Tribes, and in what was given to Moses, and Jesus and the Prophets from their Lord. We make no difference between them, and to Him are we resigned (i.e. Muslims). Whoso desireth any other religion than Islam, that religion shall never be accepted of Him, and in the next world he shall be lost." 

There are three words used by Muhammadan writers for religion, namely Din, Millah, and Mazhab, and in the Kitabu 't- Ta'rifat, the difference implied in these words is said to be as follows:- Din, as it stands in its relation to God, e.g. Dinu 'llah, the religion of God; Millah, as it stands in relation to a prophet or lawgiver, e.g. Millatu Ibrahim, the religion of Abraham; and Mazhab, as it stands in relation to the divines of Islam, e.g. Mazhab Hanafi, the religion or religious teaching of Abu Hanifah. The expression Din, however, is of general application.

Those who profess the religion of Islam are called Musalmans, Muslims, or Mu'mins. 

Ahlu 'l-Kitab, "the people of the Book," is used for Muhammadans, Jews, and Christians.