First a quick note on some linguistic things. The * means that the word is reconstructed, meaning "we don't really know, but this is a very, very, very good guess that a few dozen people, with a few dozen letters following each of their names, have come up with using historical linguistic techniques (like comparative analysis, and internal reconstruction)". A proto language" is a hypothesized language that would account for similarities between several other languages; strictly speaking it is the youngest language that contains traits of several members of a language family or subfamily.
Proto-Indo-Eutopean (PIE) is the hypothetical language that led to all Indo-European languages; it was probably highly inflected, and would have had a considerable number of crossover words that could have meant the same (or similar) thing(s). Over time, PIE broke apart (probably into dialects which would later represent individual proto languages) and formed different branches of the Indo-European language family, which in turn broke into further classifications or became extinct.
*médhu vs *mélid
All three of these words could have been used to describe what one harvested from a beehive (honey).
*mélid survived to specifically mean honey in many branches of the language family (greek, celtic languages, italic languages, anatolian languages, and even germanic languages usually evolving into the word mildew (I have no f**king clue why!)).
It is interesting to see that the etymological root word *médhu came to mean a number of things. In it's original form from PIE, it may have meant any drink made with, or having a honey-like flavor, but also could have specifically meant mead (I could describe sauternes as *médhu, but I would not call it that as it's name (adjective vs noun sort of idea)).
In hellenic languages it came to mean wine by the time of Old Ionic, and by modern it means drunkenness, and stands in ablaut relation to the word that means drunkard. It also stands as the root for the words that come to mean methanol, methylene, methyl, and methane.
Of very interesting note is the complete disappearance of this word from the italic languages, only for them to re-invent a word for it. I could list theories and cite shady evidence, but it does no good to do so. It is more interesting to see the word that the italic languages use for mead: 'water' + 'honey'. Note that I do not say 'water-honey' as a single idea, if that were the case Spanish, French, Portuguese and Catalan would all have similar phonology (sounds), instead of the obvious translation of 'water' + 'honey'.
The Germanic and Celtic languages preserve this word and the meaning almost in its entirety. Mjothr, mid, mead, met, meede and medd all descend (rather obviously) form the same historical root, *médhu, and all preserve the meaning of fermented honey, regardless of how rare the term is, or how niche the word in their respective cultures (with the older ones probably retaining it's use more than the surviving ones).