What is the definition of Beer or the Beer "family" of drinks I was picked up by someone on another SE site (unrelated) when I used the phrase "Some beers, like mead...(blah blah)" - Apparently a mead isn't a beer.  I'm good with that in the pedantic sense that it's correct, but I wasn't intending to be quite so specific, and was merely suggesting that I was not talking about spirits etc.  It got me thinking about the definition of what could, or could not be called a beer, and whether a beer is an actual drink at all, or just the name of the group of drinks that we call beers, but which really have more specific names and definitions.

The key phrase for me, in understanding it from a simple taxonomy point of view is that alcoholic drinks are essentially either Beers Wines or Spirits.  That's my starting point, which may well be rudimentary at best and wrong at worst

Using this definition, a Mead must clearly be a beer in the most basic definition; it's certainly not a win or spirit?  Obviously under beer you then have ales, lagers, ciders, perrys, meads, stouts, bitters and probably a million others, and probably further sublevels (dry cider, sweet cider, sparkling cider....), but is there a second level "beer" also at this level?

Really, is there actually a single specific drink that is a beer that is not further defined as, for example, an IPA, or a Stout?
IMHO, I would classify ciders, perrys, and meads to be more like wine than beer -- they are made by fermenting fruit or honey without substantial change to the base ingredients. Beer is made from malted grain, which must first be mashed to convert starch into fermentable sugars. Beer must contain 4 essential ingredients: Malt extract (from the grain), hops, water and yeast. It may contain others as well, but if any of these four are missing it is not beer.

I think that your premise of "that alcoholic drinks are essentially either Beers Wines or Spirits." is wrong because it oversimplifies and excludes some beverages. Besides mead and cider, there are also malt-based alcho-pop beverages (Mike's Hard Lemon, Twisted Tea, etc.) that I would not call beer either - they contain 3 of the four ingredients, but lack hops. There are other local indigenous alcoholic beverages around the world that don't fit any of these categories, like Japanese sake.

There are many styles of beer -- so the word "beer" refers to all of them, and there is no style that is called just "beer". Here is a link that describes many styles of beer, mead and cider. Note that this is not a comprehensive list; brewers keep developing new styles all the time to create desirable products. For example, Black IPAs came into vogue a couple of years ago, now I am seeing White IPAs.

BJCP Style Descriptions