Mockup is typically written as one word, without a space between the two parts of the word. This is the most common and accepted spelling of the word, and is used by dictionaries and major style guides, such as the Oxford English Dictionary and the Associated Press Stylebook. However, some sources may spell the word as two words, “mock up,” which is an alternative but less common spelling. Overall, the correct spelling of the word is “mockup,” without a space between the two parts of the word.