I'm a U.S. speaker. This is what I wrote first: "It can be either way. In modern usage, 'lowercase,' one word, unhyphenated, is the most common." But it seems this may be a U.S./British thing. According to oxforddictionaries.com, British usage is two separate words, "lower case," in a sentence like "type the words in lower case," and hyphenated when used as an adjective, "lower-case letters," while U.S. usage is the single word, "lowercase." In reality, 1) many native speakers, such as myself, would say "I'm not sure, I've seen it both ways." If it were important they would check a dictionary, or, if they were writing professionally, the appropriate style guide. 2) In English, there is a tendency for compound words to evolve--first as separate words, "lower case," then a hyphenated compound word, "lower-case," and finally a single word, "lowercase." In many cases there are many decades of overlap during which more than one form is used. 3) Using the wrong one is _not_ considered a serious error. This is amazing: Googling for examples, I actually found a perfectly serious-looking 1987 British print book on "News Reporting and Editing" that actually says modern fonts give the "freedom to put headlines in lower-case," hyphenated, and then on the same page, one paragraph later, says we recognize words by shape and "there is greater variety of word silhouette in lower case." This proves that this is a detail that writers don't pay much attention to!

HI Angel, Both can be used, I always use two words though.

P.S. In high school in the 1960s we had a "printing club" in which we set type by hand, with a composing stick. The type for a font was contained in two big divided trays called "type cases" and all the small letters were, in fact, in the lower case where they were easier to reach, and the capital letters were in the upper case.

no