Meads have styles based on recipes and ingredients. Some classifications exist within a community context for convenient labeling, others exist as a category system for competitions.
The BJCP (Beer Judge Certification Program) also has a set of guidelines for mead. These are primarily for competitions. The 2015 Guidelines will have a more complete representation, this is a summary.
Flavor note: oak is suitable for all styles unless it displaces honey as the primary flavor component. Oak that previously held another brew/spirit adds that flavor as well may change classification.
Vital Statistics for meads:
OG/ABV:
Hydromel: 1.035 - 1.080 (3.5-7.5%)
Standard: 1.080 - 1.120 (7.5-14%)
Sack: 1.120 - 1.170 (14-18%)
FG:
Dry: 0.990 - 1.010
Semi-sweet: 1.010 - 1.025
Sweet: 1.025 - 1.050
IBU: Measure of bitterness, typically relevant to braggots.
SRM: Not typically used in the beer sense. Malt is varying shades of red while honey is varying shades of yellow.
Honey is the only flavor contributor (oak/wood are the only exception). Water, yeast, non-flavor additives (nitrogen sources, specifically inactived yeasts [SIYs], fining agents, etc), or enological tannins (powdered tannins).
Non-traditional yeasts (such as Brettanomyces) and flavor or aromatics from other sources (orange peels, cinnamon, etc) do not qualify as BJCP Traditional Meads. Even flavor absorbed from spirit or wine barrels change the classification. Such may become experimental meads.
Cyser is made from apples. Pyment from grapes. Berry and stone fruit are self-explanatory. Melomel is a fruit or group of fruits that do not fit anywhere else.
M3A represents at least one fruit made with at least one spice. M3B contains one of more of the listed ingredients.
Braggots are meads containing malt (processed grains to make starches available). Hops alone do not make a braggot, they qualify as M3B. Braggots tend to mirror beer styles, but are not required to. Malt and hops tend to quickly overtake honey character
Historical Meads are historic or indigenous meads that do not fit into another category. Polish meads or Tej are notable examples.
Experimental meads are meads that fit into no other category. They may be more than two styles, atypical process or ingredients (such as cheese). It can be as simple an otherwise traditional mead aged on bourbon barrels or complex as imaginable.
These are some examples of styles within the meadmaking community not represented in the BJCP guidelines. This list is not exhaustive.
Acerglyn: Mead with maple syrup
Bochet: Mead made with caramelized honey
Capsicumel: Chili pepper mead
Sour Mead: Mead made with a culture to intentionally sour it slightly
You are the master of your brew process. As such, you are not bound by a certain guidelines. Any possibility can be explored. However, please classify any mead that fits into a category, appropriately. Mixed styles can be referred to by dominant flavor, such as a blueberry cyser having more apple character than blueberries.
The only limitation is honey is the major fermentable ingredient in mead.