Although most aircraft progress through X (experimental) and Y (prototype) prefixes, the term X by itself refers to Experimental by itself -- purely test aircraft never intended to go into mass production, e.g., X-1, X-4, X-15.
Of the pure X's, the X-15 is probably the most impressive. Some X's are largely unknown, but some were stars of their day, however briefly, such as the X-1 being first to break the sound barrier in level flight.