transformation
Transformation is the moment a cell swallows a piece of naked DNA from its surroundings and, in doing so, changes what it is genetically. A bacterium that takes up a plasmid carrying an antibiotic-resistance gene is transformed: it now has an ability it lacked a moment before.
In the lab, transformation is the key step for getting an engineered plasmid into host cells. Bacteria are usually made competent — coaxed into a receptive state — by chemical treatment or by a brief electric pulse (electroporation) that opens transient holes in the membrane. The DNA slips inside, and cells that took up the desired plasmid are then identified using a selectable marker, so only the transformed cells grow on the selection medium.
Transformation also happens in nature: some bacteria readily pick up DNA from their environment, including from dead neighbors, which is one way genes (and antibiotic resistance) spread between microbes. Historically, the discovery that a harmless bacterium could be transformed into a virulent one by DNA from dead virulent cells was a pivotal clue that DNA, not protein, is the molecule of heredity.
After a heat-shock step, a tube of bacteria is spread on a dish containing an antibiotic; only the cells that took up the resistance-carrying plasmid survive to form colonies.
Antibiotic selection lets only transformed cells grow, making them easy to find.
Transformation refers to DNA uptake by bacteria, fungi, or plant cells; introducing DNA into animal cells is usually called transfection, and delivery by a virus is called transduction.