En belleza, una "(H)elena de Troya".
Helen of Troy (from the Iliad) is widely known as "the face that launched a thousand ships." Thus, 1 millihelen is the amount of beauty needed to launch a single ship.
Negative values have also been observed—these, of course, are measured by the number of ships sunk or the number of clocks stopped. An alternative interpretation of 1 negative Helen is the amount of negative beauty (i.e. ugliness) that can launch one thousand ships the other way.
Since the original quotation was "Was this the face that launch'd a thousand ships and burnt the topless towers of Ilium", a more accurate definition of the millihelen might be the amount of beauty required to launch a single ship and ignite a wastebasket.
David Lance Goines has written a humorous article describing various Helen-units. It has a chart with the fire-lighting and ship-launching capability for different powers of "Helens". For example, a Picohelen (ph) (10^-12 helens) indicates the amount of beauty that can "Barbecue a couple of Steaks & Toss an Inner Tube Into the Pool".
Lectura entretenida jeje.