Mantra constitutes to verbs, 1) mananaat and 2) trayate. The first verb means that your mind must be continuously attracted towards a statement or a poem or a song that makes you repeat it again and again without any trace of effort. The second verb means that such a statement or poem or song must be related to God and hence, it gives you perfect protection from all corners, everywhere and everytime. 
Therefore, the definition of Mantra is a statement or a poem or a song, which is related to God and attracts your mind spontaneously to make it repeat again and again without any trace of effort. Generally, your attraction to God makes you to be attracted by any statement or poem or song that is related to God. But, apart from the attraction to God, the beauty in the meaning and the beauty in the rhythm of the statement or poem or song make an additional attraction. If both these attractions are in climax, your attraction to God becomes full. Generally, a song has highest attraction than a poem or a statement. The attraction increases gradually from statement to poem to song. Therefore, any song related to God that attracts your mind to the highest order and makes you repeat it again and again is called as Gayatri Mantra. Hence, it is said that Gayatri is the highest mantra. It means that the prayer of God in the form of a song is the highest attraction. But, people have misunderstood the actual sense and are thinking that a specific hymn related to God constructed in a specific meter called Gayatri is the highest. The hymn is not sung at all. It is simply recited. It is Gayatri because it is in the meter called Gayatri. But that is not the actual sense of Gayatri. Gayatri means any song related to God that attracts your mind. God told that He is always attracted by the song than prose and poetry (Vedaanaam saamavedosmi…).
Prose is called as the Yajurveda. Poetry is called as the Rugveda. Song is called as the Saamaveda. The verbal meaning of Gayatri actually means the song that protects you (Gaayantam traayate). It can be any song since you may like some song and some other may like some other song. Both songs are Gayatri since both protect you due to the reason that they are related to God. If the song is related to the human beings or the subjects of the human beings or to nature, it cannot protect you since the song is related to creation but not related to the creator. However, if the song describing the creation hints the greatness of the creator directly or indirectly, it can also be Gayatri. But, the description of nature or the subjects of human beings need not hint the creator always to everybody. A row of white swans was flying below the blue sky. Several people gazed that scene with excitation resulting from the beauty of the scene. But, nobody thought about the creator through that scene. Only Shri Ramakrishna Paramahamsa was excited by that scene and became unconscious thinking about the creator of the scene. Therefore, any song related to even worldly affairs can become Gayatri Mantra, if you can apply that song directly or indirectly to the God. You may sing a cinema song linking it to God in your heart. In such case, that cinema song will bring the grace of God to you and protect you. Others may think that you are attracted to the world by that cinema song. Therefore, your inner intention is going to decide everything. Even if you sing a song related to God to fulfill some worldly desire, it is not Gayatri Mantra. People may misunderstand that you are a devotee of God by hearing such divine song from your mouth. Neither you are a devotee nor is that divine song Gayatri Mantra. You may sing a cinema song of worldly meaning, but if you have directed it to God from your heart, you are a devotee and your song is Gayatri Mantra, even though people misunderstand you as a worldly person. Therefore, the public is not standard. Your intention in your heart is the standard reference for anything.
A mantra should be selected by the devotee himself or herself from the existing mantras or the devotee can convert even a non-mantra (worldly song) into mantra through the intention diverted to God. The guru is like a seller of vegetables keeping all varieties of vegetables before him and the devotee is like the customer, who visits the shop of vegetables and selects the desired vegetables.
Author: Datta Swami
