To change in form or nature — a deep, inward transformation that manifests outwardly. From meta (change) + morphe (form/essential nature). Unlike a superficial costume change, metamorphoo describes alteration of one's very essence. The English 'metamorphosis' derives directly from this word.
Used only four times in the NT, each is theologically explosive. At the Transfiguration, Jesus was metamorphoo-ed — his hidden divine glory shone through his human form (Matt 17:2; Mark 9:2). Paul applies the same verb to believers: 'be transformed (metamorphousthe) by the renewing of your mind' (Rom 12:2) and 'we are being transformed into the same image from one degree of glory to another' (2 Cor 3:18). What happened visibly to Jesus on the mountain is happening invisibly to every believer as the Spirit conforms them to Christ's image.