Oh, Webster1913, I feel it is almost blasphemous to say you are wrong. But in this case, you are. And incomplete.

Metheglin is indeed a drink of fermented honey. But it is not only that--it is, or was, a medicinal drink. The word metheglyn comes not from medd llyn--"mead liquor"--but meddyglyn: meddyg--doctor, physician; and llyn: liquid, water, though more commonly refers to a lake.*

The original metheglins were infused with medicinal herbs--cowslips, thyme, mint, etc. They were often somewhat bitter, despite being a honey-based drink, and were not taken recreationally.

One traditional recipe, reportedly from the 17th century goes:

"Take all sorts of Herbes that are good and wholesome, as Balme, Mint, Fennell, Rosemary, Angelica, Wild Tyme, Isop, Burnet, Egrimonie, and such other as you think fit; some field herbes, but you must not put in too many, especially Rosemary, or any strong Herbe; lesse than a handfull will serve of every sort. You must boyle your Herbes and straine them, and let the Liquor stand till tomorrow, and settle them. Then take off the clearest Liquor into two gallons and a halfe to one gallon and a half of honey. Let it boyle an hour, and in the boyling skin (skim) it very cleane, and set it a cooling as you do Beer. And put into the bottom of the Tub a little and a little as they doe Beer, keeping back the thick settling that lieth in the bottome of the Vessel that it is cooled in, and when it is all put together, cover it with a Cloath, and let it work very neere three days, and when you mean to put it up, skin off all the Barme cleane, put it up into the Vessel, but you must not stop your Vessel very close in three or foure dayes, but let it have all the Vent, fur it will work, but you must look to it very often, and have a peg in the top to give it vent when you heare it making a noise, as it will do, or it will break the Vessel. Sometimes I make a bag, and put in a good store of ginger sliced, some Cloves and Cinnamon, and boyle it in, and other times I put it into the Barrel and never boyle it, it is both goode, but Nutmeg and Mace do not well to my taste."

Marie Trevelyan's A Glimpse of Welsh Life and Character, 1893.

Over time, the term metheglin has changed to refer to any spiced or herb-infused mead. Personally, I like to use mulling spices--cloves, cinnamon, orange peel, etc.--to flavor mead to be drunk during Christmastime.

*About the lake: the legend goes that the Physicians of Myddfai--Meddygion Myddfai--were descended of the Shepherd of Myddfai and his wife, the Lady of the Lake. So whether the "llyn" at the end of meddyglyn just means "liquid" or is also a pun on "llyn", i.e. the lake from which the meddyion are sprung, is unclear to me.