“Lookup” is a noun, meaning “the process or instance of looking something up; especially the process of matching by computer the words of a text with material stored in memory” (Merriam-Webster Online Dictionary). The same distinction holds true for “hookup” and “hook up.” The single word is the noun.

How do you spell hooked up?

hookupan act or instance of hooking up.an assembly and connection of parts, components, or apparatus into a circuit, network, machine, or system.the circuit, network, machine, or system so formed.More items