(n.) chiefly British English name for the final letter of the English alphabet
So zed is British and zee is American, yes? Well, that might be the case today, but once upon a time things were quite different...
Historically, both zed and zee were used pretty much interchangeably in both British and American English, alongside a whole host of other more outlandish names for the last (or rather, second last) letter of the alphabet, like izzard, uzzard, zad, shard and, our personal favourite, ezod. Of the two we’re talking about here, however, zed is by far the oldest, and takes its name via French and Latin from that of its Greek equivalent, zeta.
Zed first appeared in print in the early 1400s, in a Middle English document that fairly straightforwardly described it as “þe laste lettre of þe a b c”—which is considerably nicer than what William Shakespeare had to say about it...
Zee, on the other hand, first appeared in print in a British language textbook—Thomas Lye’s New Spelling-book—in 1677. The name zee itself is thought to have originated as nothing more than a dialect variation of zed, probably influenced by the regular bee, cee, dee, ee pattern of much of the rest of the alphabet. But precisely how or why it became the predominant form in American English is unclear.
One widely-held theory is that because zed, as the older of the two, was the most widespread variation amongst British English speakers, during the Revolutionary War American English speakers looking to distance themselves from anything even vaguely British simply adopted the zee version as their own to make a stand—no matter how small it might seem—against British control. Alternatively, there mightn’t have been any political reasoning behind it at all, and the name might simply have come to the forefront as American English was forced to adapt and simplify as more and more colonists—coming from ever more distant countries, and speaking an ever more varied array of languages—began arriving in the New World.
Whatever the motivation might have been, by the mid-nineteenth century zee had become the standard form of the letter Z in the United States, and has remained so ever since.
Though the campaign to resurrect ezod starts here.