IMHO, the definition for cheese is when you execute a strategy/tactic to your opponent (or vice versa) that they are not prepare for (or less prepared). You can call these strategy "off the books" but strictly speaking, where's the "official" book that stores the "allowable" strategy. The whole point of RTS (Real Time STRATEGY) game is coming up with a strategy that beat your opponent. The best way is to beat when opponent are not expecting it (Metagame)

Some of the common "cheese" strategy, 5 rax reaper, cannon wall in at opponent's choke, planetary fortress rush, mothership rush, proxy hatchery with queen + spine, 2 gate zealot all in.

Some say that koreans are immune to cheese. You can see so many videos on youtube showing how koreans deal with cheese and counter push and win. This is because koreans, and most above average gamer would've played with these cheese before and instead of whining about it, they have the attitude to find out ways to defend against it.

Most people are mad when they get cheesed, saying things like "That stupid idiot cheesed me with a proxy pylon in my base", then I'd say "Why did you scout your base for pylon earlier instead of whining about it after?"
Same goes to all the other strategy, reaper rush can be fend of if you scout properly and defend accordingly.

Shu Tzu once said “Know thy self, know thy enemy. A thousand battles, a thousand victories.”