God is said to be love. Perhaps the best definition comes from who God is and how God relates within the blessed Trinity? The Father loves the Son, the Son obeys the Father and loves him, the Spirit does the will of God and sheds the Lord of God into the hearts of the faithful. Perhaps the love of God in the hearts of the faithful is a good indication of what love is but is it perfected in them yet?
1 John 4:7-17 NJB (7) My dear friends, let us love one another, since love is from God and everyone who loves is a child of God and knows God. (8) Whoever fails to love does not know God, because God is love. (9) This is the revelation of God's love for us, that God sent his only Son into the world that we might have life through him. (10) Love consists in this: it is not we who loved God, but God loved us and sent his Son to expiate our sins. (11) My dear friends, if God loved us so much, we too should love one another. (12) No one has ever seen God, but as long as we love one another God remains in us and his love comes to its perfection in us. (13) This is the proof that we remain in him and he in us, that he has given us a share in his Spirit. (14) We ourselves have seen and testify that the Father sent his Son as Saviour of the world. (15) Anyone who acknowledges that Jesus is the Son of God, God remains in him and he in God. (16) We have recognised for ourselves, and put our faith in, the love God has for us. God is love, and whoever remains in love remains in God and God in him. (17) Love comes to its perfection in us when we can face the Day of Judgement fearlessly, because even in this world we have become as he is.