likeness: the matching form one thing takes toward another, whether an image made to stand for it or a real sharing of the same condition
This word is built on the ordinary adjective for "like" (ὅμοιος), and its ending makes it name a product rather than a quality: not similarity in the abstract but the likeness itself, the form that matches. So it can point at something you could see, the shape a locust presents to the eye or a figure carved to stand for a god, and it can point at something no one could see, the way one death answers to another.
Most of its uses are Paul's, and he reaches for it exactly where a resemblance has to be affirmed without being collapsed into sameness. Sending the Son "in the likeness of sinful flesh" says the humanity was real while the sin was not his. Saying that believers are joined to "the likeness of his death" says their dying genuinely corresponds to his without being the same event. The word lets him hold two things at once, and readers have argued ever since about how far the likeness reaches in each place.
Because of that balance the word sits close to several others and is easy to blur with them. It is not the outward shape a thing wears for a time (σχῆμα), nor the form that expresses what something truly is (μορφή), nor the image that represents an original and can even share in it (εἰκών), nor the stamp or model a later thing is shaped by (τύπος). It is the correspondence itself, which is why the same word fits a manufactured idol and a shared human condition.
Nuances
Whether ἐν ὁμοιώματι σαρκὸς ἁμαρτίας means that the Son assumed fallen human nature itself, or a real humanity that corresponded to it while remaining sinless. The debate turns on how much reservation ὁμοίωμα carries: a qualifier that safeguards sinlessness, or a term of full solidarity.
- majorityὁμοίωμα marks a real likeness that is also a real difference: the Son took true σάρξ, but not sin, so the noun guards his sinlessness rather than diluting the incarnation.; BDAG, s.v. ὁμοίωμα; Douglas J. Moo, The Epistle to the Romans (NICNT), on Rom 8:3; Joseph A. Fitzmyer, Romans (Anchor Bible), on Rom 8:3
- minorityThe Son assumed fallen human nature as such, the σάρξ as it actually is after Adam, without ever sinning; ὁμοίωμα asserts full solidarity rather than a reservation.; C. E. B. Cranfield, The Epistle to the Romans, vol. 1 (ICC), on Rom 8:3
What τῷ ὁμοιώματι τοῦ θανάτου αὐτοῦ denotes in Rom 6:5, and how σύμφυτοι governs the dative: union with the baptismal rite that images Christ's death, or union with Christ in a death corresponding to his.
- majorityThe ὁμοίωμα is the believer's death with Christ, a dying that corresponds to his; σύμφυτοι expresses union with Christ himself rather than with a rite.; Douglas J. Moo, The Epistle to the Romans (NICNT), on Rom 6:5
- minorityThe ὁμοίωμα is baptism itself, the enacted copy or representation of Christ's death with which the believer is joined.; C. E. B. Cranfield, The Epistle to the Romans, vol. 1 (ICC), on Rom 6:5