(1) The modification of a genome by the external application of DNA from a cell of different genotype.
(2) Conversion of normal eukaryotic cells to a cancer-like state of uncontrolled division.

In eukaryotic cells, the word transformation is normally used only to describe a process that causes the cell to become malignant (cancerous). To avoid confusion, the introduction of a foreign gene that does not cause malignancy is referred to as transfection. In prokaryotic cells, the term transformation is commonly used to describe a genetic change caused by uptake of foreign DNA.