Citron
A source of confusion is that "citron" or similar words in French (and other languages), and English are false friends, as they refer to the lemon. Indeed, into the 16th century, the English name "citron" included the lemon and perhaps the lime as well.
Citrus
The Moroccan professor Henri Chapot discovered that the acidity in the more common citrons or lemons is indicated by red on the inner coat of seeds specifically on the chalazal spot, violet pigmentation on the outer side of the flower blossom, and by the new buds that are reddish-purplish. The acid-free varieties of citrus are completely lacking the red color in all the mentioned spots. This designation was cited by Herbert John Webber and Leon Dexter Batchelor, the editors of the fundamental treatise on citrus, namely The Citrus Industry, which was published by the University of California, Riverside in 1967.