The word Metta is from the Pali language - the language of the earliest Buddhist texts. It is a multi-significant term meaning:
The Pali commentators define Metta as the strong wish for the welfare and happiness of others of all social, religious, racial, political and economic status. In Metta the heart opens unconditionally encompassing all that is with acceptance, awareness, and good will. Metta is universal, finding expression in all religions and societies. Metta is an altruistic attitude of love and friendliness. Metta is devoid of self-interest. Metta is the attitude of a friend who wants to give to another the best to further their well-being. Metta makes one a pure font of well-being and safety for others. Just as a Father gives his own life to protect his child, so Metta only gives and never wants anything in return. Through Metta one refuses to be offensive or offended and renounces bitterness, resentment and animosity of every kind. In a world menaced by all kinds of destructiveness, Metta in deed, word and thought is the means to bring concord, peace and mutual understanding.
Metta: Universal Love.
“Metta is the ability to embrace all parts of ourselves, as well as all parts of the world. Practicing metta illuminates our inner integrity because it relieves us of the need to deny different aspects of ourselves.”
? Sharon Salzberg, Lovingkindness: The Revolutionary Art of Happiness
The perfection of loving-kindness is the wish to provide for the welfare and happiness of the world, accompanied by compassion and skillful means; literally it means benevolence.
Loving-kindness is mentioned immediately after the perfection of resolve:
because loving-kindness perfects the determination to undertake activity for the welfare of others;
in order to list the work of actually providing for the welfare of others right after stating the determination to do so, for "one determined upon the requisites of enlightenment abides in loving-kindness"; and
because the undertaking (of the activity for the welfare of others) proceeds imperturbably only when resolve is unshakable.
The Perfection of Loving-kindness:
~Quotes from the Theravada Tradition