Fusion is the process of forcing the nuclei of atoms so close together that they fuse into a nucleus of a new element. (Fission, of course, is the opposite: energy produced by splitting nuclei.) The new, fused nucleus weighs less than the original two.

FORBES: A Nuclear Hybrid

Sir Ian and his team fused the genetic information from an adult sheep cell nucleus with an egg cell to form an embryo that developed into a genetically identical copy of the original adult.

BBC: Dolly the sheep pioneer knighted