What, exactly, is "gene therapy"? Crystallizing a modern definition of "gene therapy" has now become critically important on several fronts. Following the recent of passage the 21st Century Cures Act, FDA now relies on the term to give regulatory incentives to sponsors of certain ‘regenerative medicines’, including ‘gene therapies’. NIH Director Francis S. Collins and FDA Commissioner Scott Gottlieb recently announced that Recombinant DNA Advisory Committee ("RAC") approval would no longer be required for "gene therapy" because "there is no longer sufficient evidence to claim that the risks of gene therapy are entirely unique and unpredictable." And in July 2018 FDA issued six draft guidance documents on "gene therapy" but none with a substantially new definition. We review the contours ofthe term here, and propose that "gene therapy"— in the lay, scientific, and legal sense — should be defined as an intentional and expected permanent alteration of a specific DNA sequence of the cellular genome — that is, the sum of DNA that exists within a cell — for a clinical purpose. We also propose that gene therapy, as we define it, can be further categorized into at least three types: direct,compensatory, and augmenting. Defining gene therapy in this fashion would provide clarity to scientists and regulators alike.