“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.

What is considered a hookup?

Hooking up means anything BUT sex. Some people use to for just making out, but usually its used for oral sex. Hooking up means any sexual act with a partner of either gender. Making out, oral sex, intercourse, manual stimulation and anal sex would all apply.