1: an act, process, or instance of transforming or being transformed 2: false hair worn especially by a woman to replace or supplement natural hair 3 a (1): the operation of changing (as by rotation or mapping) one configuration or expression into another in accordance with a mathematical rule; especially: a change of variables or coordinates in which a function of new variables or coordinates is substituted for each original variable or coordinate (2): the formula that effects a transformation b:function 5a c: an operation that converts (as by insertion, deletion, or permutation) one grammatical string (as a sentence) into another; also: a formal statement of such an operation 4: genetic modification of a bacterium by incorporation of free DNA from another bacterial cell; also: genetic modification of a cell by the uptake and incorporation of exogenous DNA