The Helen (H), not to be confused with Helen of Troy, is a unit of measurement that quantifies beauty. There are actually two separate definitions of this unit.
I’m getting all this information from Wikipedia and, more specifically, the definitions proposed by Thomas Fink, to be clear.
The first definition is that 1 Helen is enough to launch a thousand ships (each ship composed of 50 oarsmen). The second definition states that 1 Helen is the quantity of beauty to be more beautiful than 50 million women.¹⁾

The miliHelen (mH) is pretty self-explanatory as 1/1000th of a Helen: so the power to launch one ship or to be more beautiful than 50 thousand women.

The Helena (Ha) is a unit that used to be explained on Wikipedia, but which mysteriously disappeared at some point.^A) It was a logarithmic unit that focused on those 50 oarsmen. To be worth 1 more Helena, you have to be more beautiful than twice as many women as the previous one. So, for example: 1 Helena is worth 2 women. However, when you’re 2 Helenas, you are more beautiful than 4 women. However, 1 oarsman (or 1/50th of a boat) is worth 10 Helenas because a man is only willing to risk his life for a woman who is the most beautiful out of 1000.

A negative value indicates you are so ugly you beach ships.¹⁾

While I find the unit funny…I must, unfortunately, offer some criticism.
First, those 1186 ships that were launched to reclaim Helen did not do so for her beauty…but due to a binding oath. The oath specified that if anyone tried to take Helen for themselves, everyone else would team up to reclaim her. Her beauty was not a factor in this.
That means that the foundation of the unit is on shaky ground…y’know, ignoring the fact Helen herself is worth more than one Helen.
Further, this unit is very subjective in a way that’s kinda uncomfortable for actual use. For example, what happens in the event that all women are equal in beauty? The Helen, as a unit, is defined in terms of the beauty of the woman in question in relation to the women around her, right? So in the event that all women are equally beautiful, all will be 0 Helens, because none of them are more beautiful than each-other.
Thus, if we wanted to seriously define a unit of measurement for quantifying beauty, we should go about it in a different way.
That isn’t getting into the hilarious fact that Wesley, from The Princess Bride is possibly the only other character in anything to have had ships sent out for them…and even then, those ships were never sent out.