(n.) a curled symbol, &, used in writing to represent the word ‘and’
This: & is an ampersand. As a symbol, it’s derived from a handwritten combination of the letters E and T, as in et, the Latin word for “and”. You might have already known that. But whether you did or you didn’t, the fact is that the swirly thing you usually call an “and sign” actually has a name. And if it has a name, then there’s a reason for it having that name. Which is where we come in.
Up until as recently as the mid 1900s, it was standard practice when reciting the alphabet to use the Latin phrase per se (literally “by itself”) to differentiate between individual letters of the alphabet—like A, I, and O—and single-character homographic words—like a, I and O.
So the letter A would be read as “A per se A”, to ensure it was distinguished from the indefinite article a. The letter I, similarly, would be “I per se I” to differentiate it from the pronoun I. And the letter O would be “O per se O” to differentiate it from the interjection O!
But because it was just another character representing just another unit of the English language, the symbol & was also once considered a letter of the alphabet, and was included after Z in twenty-seventh place. And so to differentiate & from the conjunction and, the alphabet once ended with a final “X, Y, Z, and & per se &”.
Sometime in the early nineteenth century, this “and per se and” ran together into a single word, and, eventually, the name ampersand was born.
& that’s that.