(Thahir) is the one who regards the exterior of things without seeking knowledge of the interior reality. For example, the Thahir performs his ritual prayer in a mechanical manner, without seeking knowledge of the symbolism of the bodily postures or the dynamic and infinite meanings contained within the Sacred Words which are spoken; and the Thahir observes the fast of the Holy Month of Ramadan, but due to his ignorance of the deen of Islam and his lack of spiritual resolve the fasting results mainly in hunger and thirst. The same applies for all aspects of Islam. The 'external one' always looks at the outer forms of such things and never considers to seek their inner meanings.
Islam means peace and the person who follows this peace is called the Muslim. From another root meaning of the word, Islam means complete submission and surrender, and the person who submits and surrenders himself to the will of his Lord is the person who has accepted Allah as his Creator, Sustainer, Lord and the Master of the Day of Judgement.
So from both the definitions Islam means a way of life for people who have opted to follow peace as opposed to chaos, order as opposed to disorder, discipline as opposed to indiscipline, submission as opposed to refusal, surrender as opposed to insolence, obedience as opposed to disobedience etc. All these things combined together to define Islam as away of life by which a person becomes a follower of peace and not a follower of chaos. And the followers of peace are the Muslims whose duty it is: "to establish peace and what is good, and to prevent chaos and what is evil, and they are the best people." (Al-Qur'an 3:110)