Melomel names a mead fermented with fruit added to the must alongside honey — one entry in a whole naming taxonomy traditional mead making uses to specify exactly what got blended in, rather than lumping every fruited or flavored variation under one generic "flavored mead" label the way some other beverage traditions do.
A naming system worth knowing beyond just this one term
Cyser specifically means a mead made with apple juice or cider in place of some of the water; pyment specifically means one made with grape must or juice; metheglin specifically means one flavored with spices rather than fruit. Melomel is the broader catch-all for fruit generally, covering everything from a single-berry addition to an elaborate multi-fruit blend, where cyser and pyment name two specific, narrower fruit choices within that same broader category.
Where the actual brewing detail lives
The recipe-level detail — gravity ranges, fruit-addition timing, nutrient needs, and where homebrewers most commonly go wrong — belongs on the Melomel style guideline rather than here; this entry exists to place the term correctly within mead's wider naming vocabulary for anyone who's run across "melomel," "cyser," or "pyment" in a recipe or a competition category and wasn't sure how they relate to each other.